Fix the legacy reminder alert test race

[Aaronontheweb]

Cause

The fixture manager and the test store shared one reminder directory.

File.WriteAllText exposed a partial JSON file to the fixture store. The fixture store could classify and delete that file as invalid.

The actor under test then had no legacy rejection to report.

Fix

• Give the test a private temporary directory.
• Construct one store after the fixture write completes.
• Keep the actor health response as the startup barrier.
• Remove the redundant sink poll and obsolete comments.

Proof

• The focused test passed 20 consecutive runs.
• All 39 ReminderManagerActorTests passed.
• The file header check passed.
• Slopwatch found no new issue. It reported one existing SW004 warning in PowerShellHostProbeTests.cs.

This pull request does not change production code.

[Aaronontheweb]

LGTM