Add diagnostics support to inline chat and related actions (#296939)

- Introduce `selectionHasDiagnostics` context key to track diagnostics in the editor selection.
- Implement `FixDiagnosticsAction` to allow users to fix diagnostics directly from inline chat.
- Enhance `InlineChatController` to optionally attach diagnostics to chat requests.
- Update inline chat configuration to include `FixDiagnostics` setting.
- Register new action in the inline chat affordance for fixing diagnostics.