Encryption integration with REST scan planning

### Feature Request / Improvement

See discussion in https://github.com/apache/iceberg/pull/13225#discussion_r2739557080. Lifting out from that thread and the TODO in that PR to a separate issue.

Iceberg encryption should integrate well with scan planning, though it might require encrypting file IO wrapping in certain places. Importantly, it's worth integration testing this too which can be tricky (see https://github.com/apache/iceberg/pull/15603).

### Query engine

None

### Willingness to contribute

- [ ] 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 discussion and TODO in pull request 13225, then review the integration-testing approach in pull request 15603. Determine the scan-planning and file-IO integration points, and verify completion with integration tests covering encryption during REST scan planning.

Written by the indexing model from the issue text.