fix: continue recovered streams in a new segment

maintainer edit: streamed timeout recovery was returning the retried response internally while the channel still treated the final outbound as already streamed. End the current stream segment before retry/fallback recovery so subsequent deltas are delivered in a new segment.

tests/agent/test_runner_fallback.py

@@ -323,18 +323,24 @@ class TestFallbackOnStreamStalledAfterContent:
         )
 
         streamed: list[str] = []
+        recoveries: list[str] = []
 
         async def _delta(text: str) -> None:
             streamed.append(text)
 
+        async def _recover() -> None:
+            recoveries.append("recover")
+
         result = await fb.chat_stream(
             messages=[{"role": "user", "content": "hi"}],
             on_content_delta=_delta,
+            on_stream_recover=_recover,
         )
         assert result.finish_reason == "stop"
         assert result.content == "fallback ok"
         factory.assert_called_once_with(_fallback("fallback-a"))
-        assert "stream stalled" in streamed
+        assert streamed == ["stream stalled", "fallback ok"]
+        assert recoveries == ["recover"]
 
 
 class TestFailoverOnTransientError: