chore: Clean up the invocation context initialization logic for LIVE

1. Removed dead code: The if not run_config.response_modalities: branch was never executed since run_live already sets the default (https://github.com/google/adk-python/blob/19de45b3250d09b9ec16c45788e7d472b3e588c2/src/google/adk/runners.py#L947-L948).
2. Simplified condition: Changed from checking 'TEXT' not in ... to 'AUDIO' in ...

Co-authored-by: Xiang (Sean) Zhou <seanzhougoogle@google.com>
PiperOrigin-RevId: 853274401

Author: seanzhougoogle

src/google/adk/runners.py

@@ -1381,23 +1381,15 @@ def _new_invocation_context_for_live(
     """Creates a new invocation context for live multi-agent."""
     run_config = run_config or RunConfig()
 
-    # For live multi-agent, we need model's text transcription as context for
-    # next agent.
+    # For live multi-agents system, we need model's text transcription as
+    # context for the transferred agent.
     if self.agent.sub_agents and live_request_queue:
-      if not run_config.response_modalities:
-        # default
-        run_config.response_modalities = ['AUDIO']
-        if not run_config.output_audio_transcription:
-          run_config.output_audio_transcription = (
-              types.AudioTranscriptionConfig()
-          )
-      elif 'TEXT' not in run_config.response_modalities:
+      if 'AUDIO' in run_config.response_modalities:
         if not run_config.output_audio_transcription:
           run_config.output_audio_transcription = (
               types.AudioTranscriptionConfig()
           )
       if not run_config.input_audio_transcription:
-        # need this input transcription for agent transferring in live mode.
         run_config.input_audio_transcription = types.AudioTranscriptionConfig()
     return self._new_invocation_context(
         session,