Skip Value Expiration Check When Scanning a Tombstoned Record

[vazois]

This PR attempts to fix #1560. It also includes the following:

• Skip expiration check when SingleReader pushes a tombstoned record.
  This is safe since expiration validation will occur when reading the actual value during TransmitKeys. If the key has expired at that point, it will be skipped.
• Ensure pause reviv signal is observed under epoch protection.
• Fix ClusterMigrateTest.
  Use GarnetClientSession to ensure the correct redirection messages are generated.
• Release epoch when acquiring SuspendConfigMerge lock to prevent deadlock/livelock.

[Copilot]

Pull request overview

Fixes cluster slot migration failures when scans include tombstoned records (issue #1560) by avoiding expiration checks on tombstones, strengthening revivification pausing semantics, and updating/adding cluster migration tests to validate behavior.

Changes:

• Skip expiration validation in migration scan readers when the scanned record is a tombstone.
• Update TsavoriteKV.PauseRevivification() to coordinate pause visibility via epoch bump + signaling.
• Fix and extend cluster migration tests, including a new tombstone-focused migration test.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

File	Description
test/Garnet.test.cluster/ClusterMigrateTests.cs	Adjusts migrate test flow/expectations and adds a new tombstone migration test.
libs/storage/Tsavorite/cs/src/core/Index/Tsavorite/Tsavorite.cs	Implements epoch-coordinated revivification pause signaling and disposes the new event.
libs/cluster/Server/Migration/MigrateScanFunctions.cs	Skips expiration checks for tombstone records during scan to avoid null/value issues.

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