input: Run the context menu handler with the input released

[cesartalen]

Closes #3427

Description

A custom Input/Textarea context_menu builder ran inside the input's own update, so reading the input from it (cursor, selection) panicked and aborted the app. The handler now runs with the input released; focus bookkeeping happens in a separate update afterwards. No public API changes.

How to Test

• cargo test -p gpui-base context_menu_handler_is_deferred_and_respects_disabled
  — the handler updates its own input; panics without the fix.
• cargo test -p gpui-kit --features test-support,component --test menu
  — context menu focus and selection on inputs are unchanged.

Checklist

• I have read the CONTRIBUTING 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.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific)

[huacnlee]

Thanks for the fix, @cesartalen! #3433 opened a few minutes earlier with the same change for #3427, plus a dedicated test that reads the input from the handler, so we're going with that one and closing this as a duplicate. Your approach was correct, and we appreciate the contribution.