fix(tasks): notify workflow service when template stop bulk-stops waiting tasks

[cursor]

Bug and impact

When a user calls Stop all tasks on a template (POST .../templates/{id}/stop_all_tasks) that participates in a workflow, waiting workflow tasks are bulk-stopped in the database without going through finishRun. That means HandleWorkflowTaskCompletion is never called, and the workflow run can remain stuck in running or approval indefinitely.

Trigger scenario:

1. A workflow run queues a downstream node task (status waiting).
2. An operator uses Stop all tasks on that template from the template view.
3. The task is marked stopped in the DB, but the workflow service is never notified.
4. The workflow run never progresses to a terminal state.

Root cause

StopTasksByTemplate uses a fast path (SetWaitingTasksToStopped) for waiting tasks that bypasses TaskRunner.finishRun, which is the single completion hook that calls HandleWorkflowTaskCompletion. This interaction was introduced with the workflow feature in f29bb3f.

Fix

• Snapshot waiting workflow tasks before the bulk stop and call HandleWorkflowTaskCompletion for each after they are stopped.
• Also notify the workflow service for non-async tasks handled in the DB-only cleanup loop (same missing hook).
• Clear the dispatching flag when the dispatch goroutine exits so a crashed dispatch cannot block the runner reconciler from requeuing an undispatched task.

Validation

• Added TestTaskPool_StopTasksByTemplate_NotifiesWorkflowOnWaitingTasks
• Existing TestTaskPool_StopTasksByTemplate_DequeuesWaitingTasksByID still passes
• Reconciler tests pass