fix: add queue sentinel before raising blocking guardrail exception in streaming

Author: RVV-karma

src/agents/run.py

@@ -1097,6 +1097,7 @@ async def _start_streaming(
                         # Check if any blocking guardrail triggered and raise before starting agent.
                         for result in streamed_result.input_guardrail_results:
                             if result.output.tripwire_triggered:
+                                streamed_result._event_queue.put_nowait(QueueCompleteSentinel())
                                 raise InputGuardrailTripwireTriggered(result)
 
                     # Run parallel guardrails in background.