Change the is_resumption field in WorkflowStarted event into reason (vibe-kanban 19ac040e)

Reason should be an enumeration with only one member `resumption` currently.

Please update these part of events:

- Graph / Engine Event (GraphRunStartedEvent)
- Queue event (QueueWorkflowStartedEvent)
- SSE response event (WorkflowStartStreamResponse)

Besides, you should remove the `is_resumption` flag for `node_started` events; including:

- Queue Event (`QueueNodeStartedEvent`)
- SSE Event (`NodeStartStreamResponse`)
- Node event (`NodeRunStartedEvent`)

After finishing the changes above, adjust related tests.
You should run the affected tests and ensure they can pass. (You should use `uv run pytest` to run tests)

api/core/workflow/graph_events/graph.py

@@ -1,13 +1,16 @@
 from pydantic import Field
 
 from core.workflow.entities.pause_reason import PauseReason
+from core.workflow.entities.workflow_start_reason import WorkflowStartReason
 from core.workflow.graph_events import BaseGraphEvent
 
 
 class GraphRunStartedEvent(BaseGraphEvent):
-    # is_resumption indicating whether this `start` is a
-    # resumption of previously suspended execution.
-    is_resumption: bool = False
+    # Reason is emitted for workflow start events and is always set.
+    reason: WorkflowStartReason = Field(
+        default=WorkflowStartReason.INITIAL,
+        description="reason for workflow start",
+    )
 
 
 class GraphRunSucceededEvent(BaseGraphEvent):

api/core/workflow/graph_events/node.py

@@ -15,10 +15,6 @@ class NodeRunStartedEvent(GraphNodeEventBase):
     predecessor_node_id: str | None = None
     agent_strategy: AgentNodeStrategyInit | None = None
     start_at: datetime = Field(..., description="node start time")
-    is_resumption: bool = Field(
-        default=False,
-        description="True only when this node had already started and execution resumed after a pause.",
-    )
 
     # FIXME(-LAN-): only for ToolNode
     provider_type: str = ""