[server] fix KV snapshot restoring fallback

[zuston]

Purpose

Fix KV tablet recovery when the latest remote snapshot is broken or missing. The recovery process now correctly detects missing snapshot files reported by different filesystem implementations, removes the broken snapshot metadata, and safely falls back to the previous available snapshot.

Close #3740

Brief change log

• Detect missing snapshot data through the exception cause chain, including Hadoop's File does not exist error message.
• Distinguish newly created and restored snapshot handles so discarding a restored broken snapshot does not delete shared files referenced by older snapshots.
• Add regression coverage for broken snapshot cleanup and the latest snapshot → previous snapshot fallback flow.

Tests

API and Format

Documentation

[zuston]

PTAL @fresh-borzoni @wuchong