Fix LLMUserAggregator broadcasting mute events before StartFrame

src/pipecat/processors/aggregators/llm_response_universal.py

@@ -549,6 +549,15 @@ class LLMUserAggregator(LLMContextAggregator):
             await s.cleanup()
 
     async def _maybe_mute_frame(self, frame: Frame):
+        # Control frames must flow unconditionally — never feed them to mute
+        # strategies.  Without this guard, strategies like
+        # MuteUntilFirstBotCompleteUserMuteStrategy fire on_user_mute_started
+        # and broadcast UserMuteStartedFrame before StartFrame is pushed
+        # downstream, causing downstream processors to receive frames before
+        # StartFrame and log errors.
+        if isinstance(frame, (StartFrame, EndFrame, CancelFrame)):
+            return False
+
         should_mute_frame = self._user_is_muted and isinstance(
             frame,
             (