[dart_mcp_server] flutter_driver: expose set_frame_sync command (#392)

Fixes #391.

## Change

Uncomment `set_frame_sync` from the `flutter_driver_command` enum in `dtd.dart`. The `enabled` parameter was already present in the schema — this is a one-line change.

## Why

Apps with continuous animations (Rive, Lottie, loading spinners) keep `SchedulerBinding.transientCallbackCount > 0` indefinitely, causing `tap`, `waitFor`, and other finder-based commands to always time out. Calling `set_frame_sync` with `enabled: false` disables frame synchronization for the session, unblocking those commands — but it was unreachable through the MCP tool due to the enum restriction.