Implement HFP client (Hands-Free Unit role) for the ESP32 test harness:
Firmware:
- bt_hfp.c/h: Full HFP client with call control, audio, volume, DTMF,
voice recognition
- Enable HFP in sdkconfig.defaults with Wide Band Speech support
- Add HFP commands/events to protocol.h and cmd_dispatcher.c
Python MCP tools:
- 15 new tools: enable, connect, audio_connect, answer, reject, dial,
send_dtmf, volume, voice_recognition_start/stop, query_calls, status
- Full protocol constants in protocol.py
Tested: HFP enable returns role='hands_free_unit', ready for AG pairing
