input: Close the IME undo transaction when text is committed (#2809)

## Summary

Fixes #2761 — on macOS, undo after Chinese Pinyin IME input produced
text and caret jumps that did not match any coherent previous edit
state.

An IME composition opens an undo transaction in
`replace_and_mark_text_in_range`, but nothing closed it when the
candidate was confirmed. `replace_text_in_range` cleared
`ime_marked_range` without calling `commit_transaction`, and GPUI's
macOS layer does not send `unmarkText` after
`insertText:replacementRange:`.

The transaction therefore stayed open and every later edit merged into
the same pending change. That merge only updates `new_range`,
`new_text`, and `selection_after`, so the change kept the `old_text` and
`selection_before` of the *first* composition:

- After typing 今天 then 我们, one `Cmd-Z` restored 今天 but moved the caret
back to offset 0.
- Plain typing after an IME commit — which also arrives through
`insertText:` on macOS — was swallowed into the same group, so each undo
emitted only the last character.

Commit the transaction once the replacement is recorded. Merging within
a single composition is unchanged, so a composition is still one undo
step, and the `Atomic` push sets the coalescing boundary that keeps
later typing in its own group. `commit_transaction` is a no-op when no
transaction is open, so cut, paste, undo, and redo are unaffected.

No public API change, so no breaking changes.

## Test Plan

Two new tests, both failing before this change:

- `test_undo_manager_consecutive_compositions_are_separate_groups` — two
compositions undo and redo separately, asserting both text and caret.
- `test_undo_manager_typing_after_composition_is_a_separate_group` —
typing after an IME commit is not merged into the composition's group.

`cargo test -p gpui-base --lib` (557 passed) and `cargo test -p
gpui-component --lib` (452 passed) are green, and `cargo clippy -p
gpui-base --all-targets` is clean.

Not verified by hand: driving a real Pinyin IME through the Story app is
not scriptable, so the on-device walkthrough from the issue still needs
a manual pass.

## AI Assistance

Diagnosis, fix, and tests were written with Claude Code.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>