test(serializers): cover the Genesys AudioHook serializer

[CaptainAni187]

GenesysAudioHookSerializer is 964 lines and has no tests. It is the largest untested module under src/pipecat/, and the only telephony serializer without coverage: Twilio, Exotel and the protobuf serializer all have some.

This adds 20 tests covering the parts a Genesys session depends on.

Handshake and sequencing. open is answered with opened and marks the session open; the response echoes the client's seq as clientseq; the server's own seq increments per message; ping is answered with pong; close is answered with closed and closes the session.

Audio. A round trip through PCMU: PCM out, ulaw on the wire, PCM back at the pipeline rate, with the 8-bit to 16-bit size relationship pinned. Audio is refused in both directions before open arrives and while the session is paused.

Stereo. With ["external", "internal"] negotiated, interleaved audio is de-interleaved to the external channel only.

Control messages. dtmf becomes an InputDTMFFrame; unparseable JSON and unknown message types are ignored rather than raised; position is tracked from the client and a malformed one falls back to zero.

Outbound. InterruptionFrame becomes a barge-in event, EndFrame and CancelFrame become a disconnect, and output variables set through set_output_variables() reach the disconnect message.

Two things worth knowing

Writing these turned up behaviour that is easy to get wrong from the outside, so both are pinned by a test rather than left implicit.

_handle_open negotiates the channel from the media block in the open message and overwrites InputParams.channel. Constructing the serializer with channel=BOTH and then receiving an open that advertises ["external"] leaves it mono. The stereo test sets the channel through the handshake for that reason.

The disconnect message carries action alongside outputVariables, so the output-variables test asserts the specific key rather than the whole parameters object.

Checks

$ pytest tests/test_genesys_serializer.py -q
20 passed

$ pytest tests/test_genesys_serializer.py tests/test_exotel_serializer.py tests/test_serializers.py -q
39 passed

$ ruff check tests/test_genesys_serializer.py
All checks passed!

$ ruff format --check tests/test_genesys_serializer.py
1 file already formatted

No changelog fragment: this is tests only, with no user-facing change. Happy to add one if you would rather every PR carry one.

Everything here documents current behaviour rather than proposing changes. If any assertion pins something you consider a bug rather than the contract, say so and I will turn it into an issue instead.