input: Stop copying the value into InputPresentation (#2785)

## Description

The presentation layer split regressed a bit in memory usage, because
the value being a `String`. So each `state.presentation(cx)` would copy
the entire document, three times per render.

Rather than making that copy cheaper, the value is now gone from the
snapshot. `InputPresentation` is rebuilt on every frame, and the state
already owns the only `Rope` and hands it out through
`InputBaseState::text`, so the render path borrows it directly.
`InputPresentation` is read-only styling data again, as its doc says.

Neither of the two places that read the value needs the text itself:

- the accessibility value stays behind the `is_a11y_active()` gate, and
materializes a string only when a client is listening
- the clear button only needs the length

`selected_text` is now public, so the selection has a borrowed
counterpart to `selected_value`. The docs on both pairs point at each
other: `value` / `selected_value` hand out an owned string, `text` /
`selected_text` borrow the rope.

## How to Test

I used the alloc bench from
https://github.com/longbridge/gpui-component/pull/2783 and measured it
on the test.rs fixture repeated 20 times, after 50 draws:

  |                         | draw bytes  | draw blocks |
  |-------------------------|-------------|-------------|
  | main                    | 306,206,908 | 31,844      |
  | input-presentation-copy | 11,413,308  | 29,444      |

These numbers were measured on the `Rope` version of the snapshot.
Dropping the field removes the remaining clone as well.

## Breaking Changes

`InputPresentation::value` is removed. Read the text from the state
instead.

```diff
- let value = presentation.value();
+ let value = state.read(cx).text();
```

## Checklist

- [x] I have read the [CONTRIBUTING](../CONTRIBUTING.md) document and
followed the guidelines.
- [x] Reviewed the changes in this PR and confirmed AI generated code
(If any) is accurate.
- [x] Passed `cargo run` for story tests related to the changes.
- [ ] Tested macOS, Windows and Linux platforms performance (if the
change is platform-specific)

---------

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