fix(reminders): prevent ticks after unregister

[ReubenBond]

Azure Table reminder refreshes can complete with a stale snapshot after a reminder has been unregistered. Removing the local entry immediately allowed that stale row to recreate the reminder and deliver another tick after quiescence was reported.

Retain stopped reminders as sequence-stamped tombstones until a newer refresh confirms deletion. Unregisters without a local entry now record an identity tombstone as well, and successful re-registration replaces the stopped entry so immediate stop/restart remains supported.

Microsoft Reviewers: Open in CodeFlow

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[Copilot]

Pull request overview

This PR addresses a race where reminder table refresh results can be stale relative to an unregister operation, allowing a previously unregistered reminder to be recreated locally and fire again after quiescence was reported. It does so by retaining stopped reminders/tombstones with sequence stamps until a newer refresh confirms the deletion, while still allowing immediate stop/restart.

Changes:

• Retain unregistered reminders locally as sequence-stamped stopped entries (and add identity tombstones when no local entry exists) to suppress stale table refresh results.
• Purge tombstones on newer refresh confirmation and on range changes; clear tombstones on successful re-registration.
• Add a regression test covering stop/restart behavior before the removed reminder is purged.

Show a summary per file

File	Description
test/Orleans.Reminders.Tests/TimerTests/ReminderTests_TableGrain.cs	Adds a test for restarting a reminder after unregister but before purge.
src/Orleans.Reminders/ReminderService/LocalReminderService.cs	Implements tombstone/stopped-entry tracking and refresh suppression logic to avoid resurrecting reminders from stale reads.

Copilot's findings

Comments suppressed due to low confidence (1)

src/Orleans.Reminders/ReminderService/LocalReminderService.cs:378

• localReminders is iterated and mutated in the same loop. If any reminder is out-of-range, localReminders.Remove(...) during foreach (var r in localReminders) will throw InvalidOperationException (collection modified). The new tombstone loop correctly uses Keys.ToArray(), so the reminders loop should follow the same safe pattern.

                removedReminderTasks.Add(r.Value.StopAsync(ReminderEvents.LocalReminderStopReason.RemovedFromRange));
                localReminders.Remove(r.Key);
            }

            foreach (var key in localReminderTombstones.Keys.ToArray())

• Files reviewed: 2/2 changed files
• Comments generated: 2