fix(table): validate snapshot timestamp drift on add snapshot

[mrutunjay-kinagi]

Rationale for this change

Align snapshot timestamp validation with other Iceberg implementations by rejecting snapshots that drift backwards by more than one minute.

The new guard in AddSnapshotUpdate checks:

• snapshot timestamp vs latest snapshot_log entry
• snapshot timestamp vs table last_updated_ms

Both checks allow up to 60 seconds of clock skew tolerance.

Are these changes tested?

Yes.

• Added test_update_metadata_add_snapshot_rejects_old_timestamp_vs_snapshot_log.
• Added test_update_metadata_add_snapshot_rejects_old_timestamp_vs_last_updated.
• Verified with targeted pytest run.

Are there any user-facing changes?

No API changes.

• Invalid commit metadata now fails fast with explicit ValueError when timestamp drift exceeds tolerance.