Fix invalid IR when flattening nested `BestFitting` elements (#25398)

Summary
--

This PR fixes #25397 by dropping `StartBestFittingEntry` and
`EndBestFittingEntry` tags in
`RemoveSoftLinesBuffer`. #25144 changed `RemoveSoftLinesBuffer` to
inline the most-flat variant of
`BestFitting` elements, but it inlined them with their start and end
tags, which caused the problem
observed in #25397 when these tags became nested in another
`BestFitting` element, which interpreted
the first `End` tag as ending the outer `BestFitting` rather than the
nested one. You can see this
clearly in the IR. First, the diff between the 0.15.13 and 0.15.14 IR,
which reveals the unmatched
start tags:

<details>
<summary>0.15.13 vs main</summary>

```diff
@@ -38,29 +38,13 @@
                           "lambda ",
                           group(["arg"]),
                           ": ",
-                          best_fitting([
-                            [
-                              [
-                                <interned 1> [
-                                  "str(",
-                                  group([
-                                    indent([soft_line_break, group(["arg"])]),
-                                    soft_line_break
-                                  ]),
-                                  ")"
-                                ]
-                              ]
-                            ]
-                            [[group(expand: true, [<ref interned *1>])]]
-                            [
-                              [
-                                "(",
-                                indent([hard_line_break, <ref interned *1>]),
-                                hard_line_break,
-                                ")"
-                              ]
+                          [
+                            <interned 1> [
+                              "str(",
+                              group([indent([group(["arg"])])]),
+                              ")"
                             ]
-                          ]),
+                          ],
                           ", self.args"
                         ])
                       ])
@@ -105,27 +89,9 @@
                             "lambda ",
                             group(["arg"]),
                             ": ",
-                            best_fitting([
-                              [
-                                [
-                                  <interned 2> [
-                                    "str(",
-                                    group([
-                                      indent([soft_line_break, group(["arg"])]),
-                                      soft_line_break
-                                    ]),
-                                    ")"
-                                  ]
-                                ]
-                              ]
-                              [[group(expand: true, [<ref interned *2>])]]
-                              [
-                                [
-                                  "(",
-                                  indent([hard_line_break, <ref interned *2>]),
-                                  hard_line_break,
-                                  ")"
-                                ]
+                            [
+                              <interned 2> [
+                                "str(",
+                                group([indent([group(["arg"])])]),
+                                ")"
                               ]
-                            ]),
-                            ", self.args"
+                            ]
                           ])
+                          <START_WITHOUT_END<Group>>
                         ])
-                      ]),
-                      ")"
+                        <START_WITHOUT_END<Indent>>
+                      ])
+                      <START_WITHOUT_END<Group>>
                     ])
+                    <START_WITHOUT_END<Group>>
                   ])
-                ]),
-                ")})\""
-              ]),
-              hard_line_break,
-              ")"
+                  <START_WITHOUT_END<Indent>>
+                ])
+                <START_WITHOUT_END<Group>>
+              ])
+              <START_WITHOUT_END<Indent>>
             ])
-          ]
+            <START_WITHOUT_END<Group>>
+          ])
+          <START_WITHOUT_END<BestFittingEntry>>
         ]
       ])
     ]),
```

</details>

and then the diff between main and this PR, which shows the removed
brackets that represent the
nested tags:

<details>
<summary>main vs PR</summary>

```diff
@@ -38,12 +38,10 @@
                           "lambda ",
                           group(["arg"]),
                           ": ",
-                          [
-                            <interned 1> [
-                              "str(",
-                              group([indent([group(["arg"])])]),
-                              ")"
-                            ]
+                          <interned 1> [
+                            "str(",
+                            group([indent([group(["arg"])])]),
+                            ")"
                           ],
                           ", self.args"
                         ])
@@ -89,24 +87,15 @@
                             "lambda ",
                             group(["arg"]),
                             ": ",
-                            [
-                              <interned 2> [
-                                "str(",
-                                group([indent([group(["arg"])])]),
-                                ")"
-                              ]
-                            ]
+                            <interned 2> [
+                              "str(",
+                              group([indent([group(["arg"])])]),
+                              ")"
+                            ],
+                            ", self.args"
                           ])
-                          <START_WITHOUT_END<Group>>
                         ])
-                        <START_WITHOUT_END<Indent>>
-                      ])
-                      <START_WITHOUT_END<Group>>
+                      ]),
+                      ")"
                     ])
-                    <START_WITHOUT_END<Group>>
                   ])
-                  <START_WITHOUT_END<Indent>>
-                ])
-                <START_WITHOUT_END<Group>>
-              ])
-              <START_WITHOUT_END<Indent>>
+                ]),
+                ")})\""
+              ]),
+              hard_line_break,
+              ")"
             ])
-            <START_WITHOUT_END<Group>>
-          ])
-          <START_WITHOUT_END<BestFittingEntry>>
+          ]
         ]
       ])
     ]),
```

</details>

I think the diff between 0.15.13 and this PR is also nice to see because
it makes it a bit more
clear that the `BestFitting` element has been inlined:

<details>
<summary>0.15.13 vs this PR</summary>

```diff
@@ -38,29 +38,11 @@
                           "lambda ",
                           group(["arg"]),
                           ": ",
-                          best_fitting([
-                            [
-                              [
-                                <interned 1> [
-                                  "str(",
-                                  group([
-                                    indent([soft_line_break, group(["arg"])]),
-                                    soft_line_break
-                                  ]),
-                                  ")"
-                                ]
-                              ]
-                            ]
-                            [[group(expand: true, [<ref interned *1>])]]
-                            [
-                              [
-                                "(",
-                                indent([hard_line_break, <ref interned *1>]),
-                                hard_line_break,
-                                ")"
-                              ]
-                            ]
-                          ]),
+                          <interned 1> [
+                            "str(",
+                            group([indent([group(["arg"])])]),
+                            ")"
+                          ],
                           ", self.args"
                         ])
                       ])
@@ -105,29 +87,11 @@
                             "lambda ",
                             group(["arg"]),
                             ": ",
-                            best_fitting([
-                              [
-                                [
-                                  <interned 2> [
-                                    "str(",
-                                    group([
-                                      indent([soft_line_break, group(["arg"])]),
-                                      soft_line_break
-                                    ]),
-                                    ")"
-                                  ]
-                                ]
-                              ]
-                              [[group(expand: true, [<ref interned *2>])]]
-                              [
-                                [
-                                  "(",
-                                  indent([hard_line_break, <ref interned *2>]),
-                                  hard_line_break,
-                                  ")"
-                                ]
-                              ]
-                            ]),
+                            <interned 2> [
+                              "str(",
+                              group([indent([group(["arg"])])]),
+                              ")"
+                            ],
                             ", self.args"
                           ])
                         ])
@@ -155,7 +119,6 @@

     def __str__(self) -> str:
         return (
-            f"first stringlist of args long={list(map(lambda arg: str(
-                        arg
-                    ), self.args))})"
+            "first string"
+            f"list of args long={list(map(lambda arg: str(arg), self.args))})"
         )
```

</details>

And you can also see at the bottom here that the actual formatting that
#25144 sought to resolve
is still fixed.

Test Plan
--

New regression test based on the code in the issue