[Scheduler] Fix the race between SchedulerCallbacksTask and InvokerProcessBufferTask that lead to dropped actions

[lina-temporal]

What changed?

• SchedulerCallbacksTask must now complete for migrated schedules before GeneratorTask/InvokerProcessBufferTask are fired.

Why?

• If InvokerProcessBufferTask runs before CallbacksTask, it may operate on stale RunningStarts. Worse, InvokerProcessBufferTask will always win that race as it is an immediate pure task (runs with end of TX) vs a side effect task (async).
  • V1 scheduler migration is triggered before a V1 workflow refreshes its own running state, so it is prudent for V2 to do the refresh up-front after migration.

How did you test it?

• built
• run locally and tested manually
• covered by existing tests
• added new unit test(s)
• added new functional test(s)

Potential risks

• If CallbacksTask is stuck, no other tasks will fire for migrated workflows. Seems no worse a fate than dropping actions on the floor.