Reschedule dynamic recurring tasks when they are updated

[wintan1418]

Fixes #738

Problem

When dynamic recurring tasks are enabled, the scheduler's polling cycle (Scheduler::RecurringSchedule#reschedule_dynamic_tasks) only handles two cases:

• schedule_created_dynamic_tasks — rows whose key isn't scheduled yet
• unschedule_deleted_dynamic_tasks — scheduled entries whose key is no longer in the DB

A task whose key exists in both places falls through both checks, so any update to its schedule, class_name, arguments, queue_name, etc. is silently ignored: the scheduler keeps firing against the pre-update definition until the process restarts.

task = SolidQueue.schedule_recurring_task("import", class: "ImportJob", schedule: "0 4 * * *")
# scheduler picks it up on the next poll

task.update!(schedule: "0 8 * * *")
# expected: fires at 08:00 from now on
# actual:   keeps firing at 04:00 until the scheduler restarts

Fix

Keep the previously loaded snapshot of dynamic tasks across each polling cycle and compare every reloaded task's updated_at against it. Any task that changed gets its in-memory scheduled entry cancelled and rescheduled from the fresh record, mirroring what a delete + create with a polling cycle in between would have done.

Testing

Added a regression test that starts a scheduler with a dynamic task on an hourly cadence, updates it to every second while the scheduler is running, and asserts jobs get enqueued without a restart. The test fails on main and passes with this change.

[rosa]

Hey @wintan1418, thanks for this, but this is expected behaviour. Check this comment: #739 (comment)

[wintan1418]

Thanks @rosa — makes sense, and sorry I missed the earlier discussion on #739. Closing this one. Since it's tripped a few people now, I've opened #777 adding a short note to the README's dynamic tasks section spelling out that in-place updates aren't picked up and that unschedule + schedule again is the way to update a task.