Fix guardrails failing when interrupt_response=True

When turn_detection.interrupt_response is set to True and a guardrail triggers
an interrupt, the SDK was only sending response.interrupt but not response.cancel.
This caused the API to think the response was still active, leading to
"Conversation already has an active response" errors on subsequent requests.

Root cause:
- Guardrail interrupts are SDK-side logic, but the API doesn't know about them
- interrupt_response=True tells the API to auto-cancel on USER VOICE interrupts
- When a guardrail triggered, the SDK would skip sending response.cancel if
  interrupt_response=True, assuming the API would handle it (incorrect)

Solution:
- Add force_cancel parameter to RealtimeModelSendInterrupt
- Set force_cancel=True for guardrail interrupts
- Always send response.cancel when force_cancel=True, regardless of
  interrupt_response setting
- User voice interrupts continue to respect interrupt_response setting

Changes:
- src/agents/realtime/model_inputs.py: Add force_cancel parameter
- src/agents/realtime/session.py: Pass force_cancel=True for guardrails
- src/agents/realtime/openai_realtime.py: Check force_cancel in _send_interrupt

Fixes #1907

Author: gn00295120

src/agents/realtime/model_inputs.py

@@ -93,7 +93,17 @@ class RealtimeModelSendToolOutput:
 
 @dataclass
 class RealtimeModelSendInterrupt:
-    """Send an interrupt to the model."""
+    """Send an interrupt to the model.
+
+    Args:
+        force_cancel: If True, always send response.cancel regardless of
+            interrupt_response setting. This should be True for SDK-side interrupts
+            (e.g., guardrails) where the API doesn't know about the interrupt.
+            For user voice interrupts, this should be False (default) to allow
+            the API's automatic response cancellation when interrupt_response=True.
+    """
+
+    force_cancel: bool = False
 
 
 @dataclass

src/agents/realtime/openai_realtime.py

@@ -433,7 +433,10 @@ async def _send_interrupt(self, event: RealtimeModelSendInterrupt) -> None:
             and session.audio.input.turn_detection is not None
             and session.audio.input.turn_detection.interrupt_response is True,
         )
-        if not automatic_response_cancellation_enabled:
+        # Always cancel for force_cancel=True (e.g., guardrail interrupts)
+        # For user voice interrupts (force_cancel=False), only cancel if automatic
+        # cancellation is disabled
+        if event.force_cancel or not automatic_response_cancellation_enabled:
             await self._cancel_response()
 
         self._audio_state_tracker.on_interrupted()

src/agents/realtime/session.py

@@ -704,7 +704,9 @@ async def _run_output_guardrails(self, text: str, response_id: str) -> bool:
             )
 
             # Interrupt the model
-            await self._model.send_event(RealtimeModelSendInterrupt())
+            # Use force_cancel=True for guardrail interrupts because they are SDK-side logic
+            # and the API doesn't know about them (unlike user voice interrupts)
+            await self._model.send_event(RealtimeModelSendInterrupt(force_cancel=True))
 
             # Send guardrail triggered message
             guardrail_names = [result.guardrail.get_name() for result in triggered_results]