feat(tui) /personality (#9718)

## Summary
Adds /personality selector in the TUI, which leverages the new core
interface in #9644

Notes:
- We are doing some of our own state management for model_info loading
here, but not sure if that's ideal. open to opinions on simpler
approach, but would like to avoid blocking on a larger refactor
- Right now, the `/personality` selector just hides when the model
doesn't support it. we can update this behavior down the line

## Testing
- [x] Tested locally
- [x] Added snapshot tests

codex-rs/docs/codex_mcp_interface.md

@@ -94,6 +94,7 @@ Each response yields:
     - `reasoningEffort` – one of `minimal|low|medium|high`
     - `description` – human-friendly label for the effort
   - `defaultReasoningEffort` – suggested effort for the UI
+  - `supportsPersonality` – whether the model supports personality-specific instructions
   - `isDefault` – whether the model is recommended for most users
 - `nextCursor` – pass into the next request to continue paging (optional)