Fix: Check whether the expired snapshot is among previously finalized snapshots if the environment is not finalized

[izeigerman]

Otherwise, if the user sets the use_finalized_state=True configuration and the snapshot TTL is relatively short, they might end up in a situation where a snapshot is unavailable for diffing in the plan because it was garbage collected by the janitor process.

[Copilot]

Pull Request Overview

This PR fixes an issue where expired snapshots could be incorrectly garbage collected when use_finalized_state=True is configured and snapshot TTL is short. The fix ensures that snapshots are protected from cleanup if they are among previously finalized snapshots in non-finalized environments.

Key changes:

• Modified snapshot expiration logic to check both current and previous finalized snapshots for non-finalized environments
• Added comprehensive test coverage for the new behavior

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
sqlmesh/core/state_sync/db/snapshot.py	Updated get_expired_snapshots to protect snapshots in previous_finalized_snapshots for non-finalized environments
tests/core/state_sync/test_state_sync.py	Added test to verify expired snapshots are protected when part of previous finalized snapshots

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