input: Expose editable values to accessibility clients (#2638)

Closes #2636

### Description

Expose the current value of ordinary editable inputs and handle
AccessKit `SetValue` by replacing the full input value through the
existing state path.

Masked, password, and new-password inputs do not expose their value.
They still accept `SetValue` while enabled, which preserves write-only
password behavior. Disabled inputs do not register the write action.

AccessKit maps string value writes on Windows and macOS. Its Unix
adapter does not currently implement AT-SPI EditableText writes, so this
PR does not claim live Linux value replacement.

### Screenshot

Not applicable. Rendering does not change.

### How to Test

Exact candidate `61fa84a6` on Rust 1.97.1:

- `cargo test -p gpui-component input --lib`: 5 passed
- `cargo test -p gpui-component`: 345 passed
- `cargo clippy -p gpui-component --all-targets -- -D warnings`
- story application compile: passed

The rendered-element regression writes the actual AccessKit node, proves
that the ordinary value and `SetValue` support are emitted, sends the
exact `ActionData::Value` body through the handler, and verifies the
stored value changes. Separate cases prove masked and password values
remain hidden.

### AI Assistance

The implementation and tests were AI-assisted by Friday. Chris Hynes
defined the behavior and owns the final review.

### Checklist

- [x] I have read the [CONTRIBUTING](../CONTRIBUTING.md) document and
followed the guidelines.
- [ ] Reviewed the changes in this PR and confirmed AI generated code
(If any) is accurate.
- [ ] Passed `cargo run` for story tests related to the changes. The
story application compiles; it has not been launched.
- [ ] Tested macOS, Windows and Linux platforms performance (if the
change is platform-specific). The shared AccessKit node is tested; live
adapter behavior is not claimed beyond current AccessKit support.

---------

Co-authored-by: Friday <260232009+buildfriday@users.noreply.github.com>
Co-authored-by: Floyd Wang <gassnake999@gmail.com>