Python: Title: DevUI Fix WorkflowFailedEvent error extraction (#2706)

* Title: Fix WorkflowFailedEvent error extraction to use details instead of error Body:
Summary
Fixed WorkflowFailedEvent mapping to extract error message from details.message instead of non-existent error attribute
Added support for including details.extra context in error messages when present
Problem
The WorkflowFailedEvent handler in _mapper.py was reading event.error, but WorkflowFailedEvent uses a details attribute (of type WorkflowErrorDetails), not error. This caused all workflow failures to display "Unknown error" in the UI instead of the actual error message.
Fix
Updated the handler to match the pattern already used by ExecutorFailedEvent:
Read from event.details instead of event.error
Extract details.message for the error text
Include details.extra context when available

* improve error handling consistency

python/packages/devui/tests/test_mapper.py

@@ -596,6 +596,31 @@ async def test_workflow_failed_event(mapper: MessageMapper, test_request: AgentF
     response = failed_events[0].response
     assert response.status == "failed"
     assert response.error is not None
+    # Verify error message is correctly extracted from details.message (not "Unknown error")
+    assert "Workflow failed due to test error" in response.error.message
+    assert "Unknown error" not in response.error.message
+
+
+async def test_workflow_failed_event_with_extra(mapper: MessageMapper, test_request: AgentFrameworkRequest) -> None:
+    """Test WorkflowFailedEvent includes extra context when available."""
+    from agent_framework._workflows._events import WorkflowErrorDetails, WorkflowFailedEvent
+
+    details = WorkflowErrorDetails(
+        error_type="ValidationError",
+        message="Input validation failed",
+        executor_id="validation_executor",
+        extra={"field": "email", "reason": "invalid format"},
+    )
+    event = WorkflowFailedEvent(details=details)
+    events = await mapper.convert_event(event, test_request)
+
+    assert len(events) == 1
+    assert events[0].type == "response.failed"
+    response = events[0].response
+    # Verify both the message and extra context are included
+    assert "Input validation failed" in response.error.message
+    assert "extra:" in response.error.message
+    assert "email" in response.error.message
 
 
 async def test_workflow_failed_event_with_traceback(mapper: MessageMapper, test_request: AgentFrameworkRequest) -> None: