[DEV] Rework SimpleSetupCluster in cluster tests, and longer timeouts for select tests

[kevin-montrose]

dev version of #1762

[Copilot]

Pull request overview

This PR updates the cluster test infrastructure to use an async, more stepwise SimpleSetupCluster flow (with additional stabilization checks) and adjusts timeouts for a few long-running tests, improving reliability and debuggability of CI cluster test runs.

Changes:

• Convert cluster test setup and several test cases to async flows (including new *Async helpers).
• Add/consume [CancelAfter] timeouts during ClusterTestContext.Setup and extend timeouts for select cluster/vector tests.
• Make replication recovery logging resilient to a null logger.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
test/Garnet.test.cluster/VectorSets/ClusterVectorSetTests.cs	Converts tests and local setup helper to async and adds longer [CancelAfter] for a few slow tests.
test/Garnet.test.cluster/ReplicationTests/ClusterReplicationBaseTests.cs	Converts select tests and waits to async (WaitForConnectedReplicaCountAsync, ReconnectAsync, etc.).
test/Garnet.test.cluster/ClusterTestUtils.cs	Introduces async cluster setup + async wrappers for several cluster operations and additional post-step stabilization checks.
test/Garnet.test.cluster/ClusterTestContext.cs	Makes setup respect [CancelAfter] timeout and converts attach/sync helper to async.
test/Garnet.test.cluster/ClusterNegativeTests.cs	Converts a few replication/failover tests to async and awaits new async wait helpers.
test/Garnet.test.cluster/ClusterMigrateTests.cs	Converts one migrate test to async and awaits async node selection helper.
test/Garnet.test.cluster/ClusterConfigTests.cs	Removes a [CancelAfter] on one test.
libs/cluster/Server/Replication/ReplicationManager.cs	Null-checks logger in replication recovery logs.

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