feat(api): publish a message_replace event with saved temporary answer

This ensures that the message content in frontend UI is properly
restored.

api/services/workflow_event_snapshot_service.py

@@ -14,6 +14,7 @@ from sqlalchemy.orm import Session, sessionmaker
 
 from core.app.apps.message_generator import MessageGenerator
 from core.app.entities.task_entities import (
+    MessageReplaceStreamResponse,
     NodeFinishStreamResponse,
     NodeStartStreamResponse,
     StreamEvent,
@@ -39,6 +40,7 @@ class MessageContext:
     conversation_id: str
     message_id: str
     created_at: int
+    answer: str | None = None
 
 
 @dataclass
@@ -157,6 +159,7 @@ def _get_message_context(session_maker: sessionmaker[Session], workflow_run_id:
             conversation_id=message.conversation_id,
             message_id=message.id,
             created_at=created_at,
+            answer=message.answer,
         )
 
 
@@ -208,6 +211,11 @@ def _build_snapshot_events(
     _apply_message_context(workflow_started, message_context)
     events.append(workflow_started)
 
+    if message_context is not None and message_context.answer is not None:
+        message_replace = _build_message_replace_event(task_id=task_id, answer=message_context.answer)
+        _apply_message_context(message_replace, message_context)
+        events.append(message_replace)
+
     for snapshot in node_snapshots:
         node_started = _build_node_started_event(
             workflow_run_id=workflow_run.id,
@@ -261,6 +269,17 @@ def _build_workflow_started_event(
     return payload
 
 
+def _build_message_replace_event(*, task_id: str, answer: str) -> dict[str, Any]:
+    response = MessageReplaceStreamResponse(
+        task_id=task_id,
+        answer=answer,
+        reason="",
+    )
+    payload = response.model_dump(mode="json")
+    payload["event"] = response.event.value
+    return payload
+
+
 def _build_node_started_event(
     *,
     workflow_run_id: str,

api/tests/unit_tests/services/workflow/test_workflow_event_snapshot_service.py

@@ -175,6 +175,7 @@ def test_build_snapshot_events_applies_message_context() -> None:
         conversation_id="conv-1",
         message_id="msg-1",
         created_at=1700000000,
+        answer="snapshot message",
     )
 
     events = _build_snapshot_events(
@@ -186,6 +187,13 @@ def test_build_snapshot_events_applies_message_context() -> None:
         resumption_context=None,
     )
 
+    assert [event["event"] for event in events] == [
+        "workflow_started",
+        "message_replace",
+        "node_started",
+        "node_finished",
+    ]
+    assert events[1]["answer"] == "snapshot message"
     for event in events:
         assert event["conversation_id"] == "conv-1"
         assert event["message_id"] == "msg-1"