Proposal: ValidateTableIntegrity action

## Proposal

Add a ValidateTableIntegrity action that verifies the referential integrity of an Iceberg table's metadata — that every metadata, data, and delete file the table's metadata references actually exists at its stated location, or at a corresponding rewritten location when a destination is configured.

The action walks the full metadata graph — metadata.json log entries, manifest lists, manifests, data files, delete files (including V3 deletion vectors), statistics files, and partition statistics — verifying every referenced file exists at the checked location.

The action is standalone: the same interface serves a self-audit on a single table and a source-vs-destination check between two tables.

Proposal Design Doc : https://docs.google.com/document/d/13dMjNMqdumFGPl6OfdaMAomk9mWzxkXQwwjm9IA6QjU/edit?usp=sharing

Tracked across three sequential PRs:

- [ ] API contract (interface + factory + Result base)
- [ ] Spark implementation + core helpers + exception
- [ ] validate_table_integrity Spark procedure + user docs

## Two supported shapes

- Self-audit : Single table, no destination. Walks the metadata graph and verifies each referenced file exists at its stated location. Fits post-rollback verification, catalog audits, and detecting storage-side drift (files removed out-of-band while metadata still references them).

- Source-vs-destination : Walks source metadata, applies the configured prefix rewrite, and verifies each referenced file exists at the destination. Fits pre-registration copy verification, DR sync, migration audits, and any out-of-band copy audit (backups, Distcp output, manual file copies).

## Out of scope

- Content equivalence : validates file existence, not file content.
- Metadata internal consistency : the validator checks that referenced files exist, not that the metadata graph is otherwise internally consistent (e.g., manifest schema validity, snapshot log integrity, statistics correctness).

## Risk

- Low risk — additive, opt-in feature with no behavioral change for existing tables or actions.
- New action; does not modify any existing code paths.
- Read-only at the checked location — only performs file existence checks, never writes.

## Query engine

Spark

## Willingness to contribute

- [x] I can contribute this improvement/feature independently
- [ ] I would be willing to contribute this improvement/feature with guidance from the Iceberg community
- [ ] I cannot contribute this improvement/feature at this time

Start with the linked design document and the API contract stage. Then work through the planned Spark implementation, core helpers, exception, procedure, and user documentation; done means the three sequential stages are completed for both self-audit and source-vs-destination validation.

Written by the indexing model from the issue text.