fix: Set default response modality to AUDIO in run_session

Some native audio models require the modality to be set, so we default to AUDIO if not specified in `RunConfig`. PiperOrigin-RevId: 815952039

Author: hangfei

src/google/adk/runners.py

@@ -617,6 +617,10 @@ async def run_live(
  Either `session` or both `user_id` and `session_id` must be provided.
  """
  run_config = run_config or RunConfig()
+ # Some native audio models requires the modality to be set. So we set it to
+ # AUDIO by default.
+ if run_config.response_modalities is None:
+ run_config.response_modalities = ['AUDIO']
  if session is None and (user_id is None or session_id is None):
  raise ValueError(
  'Either session or user_id and session_id must be provided.'