Re-inject turn completion instructions after LLM context reset

When filter_incomplete_user_turns is enabled and an LLMMessagesUpdateFrame
replaces the context via set_messages(), the turn completion instructions
system message was lost. This caused the LLM to stop emitting turn
completion markers. Re-inject the instructions after set_messages() to
fix this.

src/pipecat/processors/aggregators/llm_response_universal.py

@@ -642,6 +642,9 @@ class LLMUserAggregator(LLMContextAggregator):
 
     async def _handle_llm_messages_update(self, frame: LLMMessagesUpdateFrame):
         self.set_messages(frame.messages)
+        if self._params.filter_incomplete_user_turns:
+            config = self._params.user_turn_completion_config or UserTurnCompletionConfig()
+            self._context.add_message({"role": "system", "content": config.completion_instructions})
         if frame.run_llm:
             await self.push_context_frame()