snap: keep snapshot db files until they have been applied

[darshil929]

When a follower receives two snapshots in a short period, the raft loop persisting the newer one releases all older snapshot database files, while the apply loop may still be waiting to apply an older snapshot. Its .snap.db file gets deleted while the apply is pending, and when applySnapshot opens it through OpenSnapshotBackend the server panics with "failed to open snapshot backend". This has been hit in robustness testing CI and reproduced by Antithesis on default configuration.

The Snapshotter now tracks snapshot database files that are pending apply: SaveDBFrom marks the index on save, ReleaseSnapDBs skips marked files, and OpenSnapshotBackend drops the mark once the rename consumes the file. The tracking is in memory only, so stale files from a previous run are still cleaned up at boot, same as before.

Added a test reproducing the sequence: save the db file for snapshot A, run ReleaseSnapDBs for a newer snapshot B, then look up A's file as the apply path does. It fails on main with "snap: snapshot file doesn't exist", the exact error behind the panic, and passes with the change. The test also verifies the file becomes eligible for cleanup again once the apply completes.

Fixes #18055

[kubernetes-prow]

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[kubernetes-prow]

Hi @darshil929. Thanks for your PR.

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