Fix Google STT Word‑timings error.

[itskyf]

Error

When using google.TTS(model="chirp_2", enable_word_time_offsets=False), since there is no word offsets, the resp.results[0].alternatives[0].words[0].start_offset call in _recognize_response_to_speech_event raise an error.

Traceback (most recent call last):
  File "agents/livekit-plugins/livekit-plugins-google/livekit/plugins/google/stt.py", line 267, in _recognize_impl
    return _recognize_response_to_speech_event(raw)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "agents/livekit-plugins/livekit-plugins-google/livekit/plugins/google/stt.py", line 565, in _recognize_response_to_speech_event
    start_offset = resp.results[0].alternatives[0].words[0].start_offset
                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^
  File ".venv/lib/python3.12/site-packages/proto/marshal/collections/repeated.py", line 130, in __getitem__
    return self._marshal.to_python(self._pb_type, self.pb[key])
                                                  ~~~~~~~^^^^^
IndexError: list index out of range

Related links

From Google Chirp 2: "Word-timings (Timestamps): Automatically generated by the model and can be optionally enabled. It is possible that transcrption quality and speed will degrade slightly."

[CLAassistant]

All committers have signed the CLA.

[longcw]

thanks! could you sign the CLA before we merging?

[itskyf]

@longcw Sure, I have signed the CLA