fix(gateway): guard pending_event.channel_prompt against None in recursive _run_agent

Initialize next_channel_prompt before the pending_event check and use
getattr with None default, matching the existing pattern for
next_source/next_message/next_message_id. Prevents AttributeError
when pending_event is None (interrupt path).

Cherry-picked from #10953 by @jackjin1997.

tests/gateway/test_pending_event_none.py

@@ -0,0 +1,42 @@
+"""Tests for the pending_event None guard in recursive _run_agent calls.
+
+When pending_event is None (Path B: pending comes from interrupt_message),
+accessing pending_event.channel_prompt previously raised AttributeError.
+This verifies the fix: channel_prompt is captured inside the
+`if pending_event is not None:` block and falls back to None otherwise.
+"""
+
+from types import SimpleNamespace
+
+
+def _extract_channel_prompt(pending_event):
+    """Reproduce the fixed logic from gateway/run.py.
+
+    Mirrors the variable-capture pattern used before the recursive
+    _run_agent call so we can test both paths without a full runner.
+    """
+    next_channel_prompt = None
+    if pending_event is not None:
+        next_channel_prompt = getattr(pending_event, "channel_prompt", None)
+    return next_channel_prompt
+
+
+class TestPendingEventNoneChannelPrompt:
+    """Guard against AttributeError when pending_event is None."""
+
+    def test_none_pending_event_returns_none_channel_prompt(self):
+        """Path B: pending_event is None — must not raise AttributeError."""
+        result = _extract_channel_prompt(None)
+        assert result is None
+
+    def test_pending_event_with_channel_prompt_passes_through(self):
+        """Path A: pending_event present — channel_prompt is forwarded."""
+        event = SimpleNamespace(channel_prompt="You are a helpful bot.")
+        result = _extract_channel_prompt(event)
+        assert result == "You are a helpful bot."
+
+    def test_pending_event_without_channel_prompt_returns_none(self):
+        """Path A: pending_event present but has no channel_prompt attribute."""
+        event = SimpleNamespace()
+        result = _extract_channel_prompt(event)
+        assert result is None