Allow cancel out of the streaming result (#579)

Fix for #574 

@rm-openai I'm not sure how to add a test within the repo but I have
pasted a test script below that seems to work

```python
import asyncio
from openai.types.responses import ResponseTextDeltaEvent
from agents import Agent, Runner

async def main():
    agent = Agent(
        name="Joker",
        instructions="You are a helpful assistant.",
    )

    result = Runner.run_streamed(agent, input="Please tell me 5 jokes.")
    num_visible_event = 0
    async for event in result.stream_events():
        if event.type == "raw_response_event" and isinstance(event.data, ResponseTextDeltaEvent):
            print(event.data.delta, end="", flush=True)
            num_visible_event += 1
            print(num_visible_event)
            if num_visible_event == 3:
                result.cancel()


if __name__ == "__main__":
    asyncio.run(main())
````

src/agents/result.py

@@ -75,7 +75,9 @@ class RunResultBase(abc.ABC):
 
     def to_input_list(self) -> list[TResponseInputItem]:
         """Creates a new input list, merging the original input with all the new items generated."""
-        original_items: list[TResponseInputItem] = ItemHelpers.input_to_new_input_list(self.input)
+        original_items: list[TResponseInputItem] = ItemHelpers.input_to_new_input_list(
+            self.input
+        )
         new_items = [item.to_input_item() for item in self.new_items]
 
         return original_items + new_items
@@ -152,6 +154,18 @@ class RunResultStreaming(RunResultBase):
         """
         return self.current_agent
 
+    def cancel(self) -> None:
+        """Cancels the streaming run, stopping all background tasks and marking the run as
+        complete."""
+        self._cleanup_tasks()  # Cancel all running tasks
+        self.is_complete = True  # Mark the run as complete to stop event streaming
+
+        # Optionally, clear the event queue to prevent processing stale events
+        while not self._event_queue.empty():
+            self._event_queue.get_nowait()
+        while not self._input_guardrail_queue.empty():
+            self._input_guardrail_queue.get_nowait()
+
     async def stream_events(self) -> AsyncIterator[StreamEvent]:
         """Stream deltas for new items as they are generated. We're using the types from the
         OpenAI Responses API, so these are semantic events: each event has a `type` field that
@@ -192,13 +206,17 @@ class RunResultStreaming(RunResultBase):
 
     def _check_errors(self):
         if self.current_turn > self.max_turns:
-            self._stored_exception = MaxTurnsExceeded(f"Max turns ({self.max_turns}) exceeded")
+            self._stored_exception = MaxTurnsExceeded(
+                f"Max turns ({self.max_turns}) exceeded"
+            )
 
         # Fetch all the completed guardrail results from the queue and raise if needed
         while not self._input_guardrail_queue.empty():
             guardrail_result = self._input_guardrail_queue.get_nowait()
             if guardrail_result.output.tripwire_triggered:
-                self._stored_exception = InputGuardrailTripwireTriggered(guardrail_result)
+                self._stored_exception = InputGuardrailTripwireTriggered(
+                    guardrail_result
+                )
 
         # Check the tasks for any exceptions
         if self._run_impl_task and self._run_impl_task.done():

tests/test_cancel_streaming.py

@@ -0,0 +1,22 @@
+import pytest
+
+from agents import Agent, Runner
+
+from .fake_model import FakeModel
+
+
+@pytest.mark.asyncio
+async def test_joker_streamed_jokes_with_cancel():
+    model = FakeModel()
+    agent = Agent(name="Joker", model=model)
+
+    result = Runner.run_streamed(agent, input="Please tell me 5 jokes.")
+    num_events = 0
+    stop_after = 1  # There are two that the model gives back.
+
+    async for _event in result.stream_events():
+        num_events += 1
+        if num_events == 1:
+            result.cancel()
+
+    assert num_events == 1, f"Expected {stop_after} visible events, but got {num_events}"