fix(realtime): preserve transport audio formats during agent handoff

During agent handoff in _handle_tool_call, _get_updated_model_settings_from_agent
was called with starting_settings=None, causing session.update to reset
input_audio_format and output_audio_format back to pcm16 defaults.

This overwrote transport-configured audio formats (e.g. g711_ulaw for Twilio),
closing the Twilio session and sending noise over the phone.

Fix: pass starting_settings=self._model_config.get("initial_model_settings", None)
to mirror the behaviour of __aenter__, which correctly preserves audio formats
on initial connect.

Fixes #1632

Author: xodn348

src/agents/realtime/session.py

@@ -693,9 +693,11 @@ async def _handle_tool_call(
             # Update current agent
             self._current_agent = result
 
-            # Get updated model settings from new agent
+            # Get updated model settings from new agent, preserving transport-configured
+            # settings (e.g. audio format) from initial_model_settings so they are not
+            # reset to defaults during the handoff session update.
             updated_settings = await self._get_updated_model_settings_from_agent(
-                starting_settings=None,
+                starting_settings=self._model_config.get("initial_model_settings", None),
                 agent=self._current_agent,
             )