test(client): synchronize dynamic reconnect replacement timer

[SunSi12138]

Refs #387.

New record analyzed

Nightly Regression #285 reported SharpLinkReconnectPolicyLifecycleRaceTests.DynamicGenerationReplacementShouldRetireTheOldReconnectOwnerAcrossPolicyWake on macOS. After generation 2 was materialized / snapshot-processed, the test advanced manual time by 999ms + 1ms but observed old=1, new=0.

Static root cause

This is a test synchronization race around reconnect-policy wake, not a demonstrated production ownership failure.

DynamicClusterReconnectCoordinator.EnsureReconnect starts ReconnectAsync synchronously up to its first reconnect-delay await. If generation replacement is processed just before UpdateReconnectPolicy, the replacement owner can first arm the old policy's 10-second timer. The policy update then cancels that generation's delay; re-entering the reconnect loop and arming the new 1-second timer happens on the async cancellation continuation.

PublishedSnapshotProcessed proves topology application plus the initial reconnect reconciliation returned, but it does not prove that this policy-wake continuation has run. Likewise, CreatedTimerCount > beforeReplacement can be satisfied by the now-cancelled 10-second timer. The test can therefore advance fake time to t=1 before the live one-second timer exists; if the continuation then arms the timer at t=1, it is due at t=2 and the test never advances again.

Fix

Use the test-local ManualTimeProvider.WaitForPendingTimerAsync(TimeSpan.FromSeconds(1)) seam, already present in this file for the analogous reconnect timer-registration race, before advancing fake time. This requires a live timer at exactly the current manual timestamp + one second and cannot be satisfied by the cancelled old-policy 10-second wait.

The existing assertions remain intact:

• no replacement dial before 1 second;
• replacement dial at the 1-second boundary;
• retired old endpoint never dials again;
• current generation owns the subsequent reconnect attempt.

No production code, reconnect semantics, timeout widening, retries, sleeps, or workflow changes.

Validation

Static analysis / diff review locally; executable validation is delegated to PR CI.