dialog: make the close button accessible

[grishy]

The dialog’s close button has no accessible name or AXPress action on macOS. This patch gives the button a localized name and moves the existing Cancel handler from its wrapper onto the button.

Native accessibility dumps of the same close button, before and after (unrelated fields omitted):

 {
   "role": "AXButton",
-  "title": "",
+  "title": "Close",
   "enabled": "1",
-  "actions": []
+  "actions": ["AXPress"]
 }

Appearance and public API are unchanged. The name uses Dialog.close (need to check non-English versions). Closing still goes through Cancel, including the on_cancel callback and its ability to prevent dismissal.

Screenshot

How to Test

MTL_HUD_ENABLED=1 ./script/run-story-macos Dialog

1. Open Custom Width (400px).
2. Use Xcode’s Accessibility Inspector to inspect the top-right close button: AXButton, name Close, action AXPress.
3. Perform AXPress: the dialog should close.
4. Reopen, focus Close, and press Space: it should close again.

Use Space for button activation: the existing Dialog Enter binding invokes Confirm instead, a separate keyboard-routing issue. Didn't fix here.

Code checks

cargo test -p gpui-base -p gpui-component --lib --locked
cargo clippy -p gpui-base -p gpui-component -p gpui-component-story --all-targets --locked -- -D warnings
rustfmt --check --edition 2024 crates/component/src/dialog/dialog.rs

771 base and 418 component tests passed, along with strict Clippy and formatting. No new Rust test was added; the close-button regression was checked through native accessibility.

Checklist

• Read CONTRIBUTING.md.
• Human-reviewed the patch, including the AI-assisted changes.
• Manually tested the patch as a human reviewer.
• Windows and Linux runtime validation.

AI assistance

OpenAI’s gpt-6-astra helped prepare the patch, native verification scripts, and this description. I reviewed the changes; the checks reported above were run by the agent and myself also initially

[grishy]

Also in macOS xcode