test(openai): add comprehensive tests and widget state support

- Add E2E tests for OpenAI transport with mocked window.openai
- Add error handling tests for all API methods (callTool, sendMessage, etc.)
- Add widget state tests (ui/update-model-context ↔ setWidgetState)
- Add content format handling tests (arrays, structured content, etc.)
- Add host context extraction tests (userAgent, safeAreaInsets, viewport)
- Implement ui/update-model-context → setWidgetState mapping
- Deliver widgetState as ui/notifications/widget-state notification
- Document unsupported features in JSDoc (uploadFile, requestModal, etc.)

Author: ochafik