Enhanced GraphEngine Pause Handling (#28196)

This commit: 

1. Convert `pause_reason` to `pause_reasons` in `GraphExecution` and relevant classes. Change the field from a scalar value to a list that can contain multiple `PauseReason` objects, ensuring all pause events are properly captured.
2. Introduce a new `WorkflowPauseReason` model to record reasons associated with a specific `WorkflowPause`.

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: -LAN- <laipz8200@outlook.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

api/core/app/layers/pause_state_persist_layer.py

@@ -118,6 +118,7 @@ class PauseStatePersistenceLayer(GraphEngineLayer):
             workflow_run_id=workflow_run_id,
             state_owner_user_id=self._state_owner_user_id,
             state=state.dumps(),
+            pause_reasons=event.reasons,
         )
 
     def on_graph_end(self, error: Exception | None) -> None: