test: add emit_exchange_error_event to MockChatBridge

uipath-runtime 0.10.x added this method to UiPathChatProtocol; the test
mock was missing it, breaking mypy on the bumped runtime.

Author: radu-mocanu

packages/uipath-agent-framework/tests/test_hitl_e2e.py

@@ -87,6 +87,9 @@ async def emit_interrupt_event(self, resume_trigger: UiPathResumeTrigger) -> Non
     async def emit_exchange_end_event(self) -> None:
         pass
 
+    async def emit_exchange_error_event(self, error: Exception) -> None:
+        pass
+
     async def wait_for_resume(self) -> dict[str, Any]:
         """Return CAS-format approval/rejection response."""
         return {