Use the reminder actor clock in the expiry test

[Aaronontheweb]

Summary

• Use the reminder actor's FakeTimeProvider in the expiry test.
• Remove the comparison between a frozen actor clock and the system clock.

Cause

PR #1766 exposed this race in the macOS CI job.

The test created ExpiresAt from TimeProvider.System. The actor compared that value against its injected FakeTimeProvider.

Clock drift could place the test expiry after the actor's frozen time. The actor then started an execution instead of disabling the reminder.

The fix derives the test time from the same fake clock. It does not increase a timeout or add a delay.

Validation

• The focused test passed 20 consecutive runs.
• All 2,759 Netclaw.Actors tests passed.
• The full solution test run passed. Opt-in integration tests skipped as configured.
• dotnet slopwatch analyze passed with zero issues.
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify passed.
• git diff --check passed.