- Remove synthetic "default" thinking option ID, use concrete option IDs from providers
- Normalize thinking option IDs to undefined when "default" or empty string
- Update app UI to display "Model default" label for fallback thinking option
- Fix Codex thinking option round-trip through app-server turn context
- Add test for thinking option ID persistence across Codex sessions
- Update test helpers to use actual default thinking option IDs
- Update OpenCode label consistency to match app display logic