getMTU() conformance test

Targeted conformance for BluetoothRemoteGATTCharacteristic.getMTU()

Runs an explicit pass/fail check sequence against the new API added by CL 7879985. Designed to be the one-shot test you can point reviewers at to verify the API on each platform.

Required launch flag:

--enable-features=NewBLEGattSessionHandling,WebBluetooth

Requires the dino tester ESP32-C3 firmware advertising on FFE0/FFE1.