Handle log truncation in Delta to Iceberg incremental sync

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/incubator-xtable/issues?q=is%3Aissue) and found no similar issues.

### Please describe the bug 🐞

There appears to be a discrepancy between how incremental‑sync safety is validated for Delta vs. Iceberg when historical metadata has been truncated. Iceberg correctly detects expired snapshots and triggers a full snapshot sync, but Delta’s `isIncrementalSyncSafeFrom` only checks whether a commit existed at the given instant—not whether the underlying log files still exist. As a result, incremental sync is incorrectly marked “safe,” leading to failures such as `DeltaFileNotFoundException` when attempting to reconstruct old commits. This issue proposes enhancing Delta’s safety check to verify that required commit files are still available before approving incremental sync.

### Are you willing to submit PR?

- [x] I am willing to submit a PR!
- [ ] I am willing to submit a PR but need help getting started!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

Start with Delta's isIncrementalSyncSafeFrom check and compare it with Iceberg's handling of expired snapshots. Verify how required historical commit files are checked, then confirm that missing log files trigger a full snapshot sync instead of a DeltaFileNotFoundException.

Written by the indexing model from the issue text.