feat(iceberg): add delete file support to SnapshotProducer

This enables SnapshotProducer to accept and process delete files
(position deletes and equality deletes) alongside data files.

Changes:
- Add added_delete_files field to SnapshotProducer
- Add validate_added_delete_files() for delete file validation:
  - Rejects delete files in V1 format
  - Validates content types (PositionDeletes, EqualityDeletes)
  - Requires equality_ids for equality delete files
  - Validates partition spec compatibility
- Add write_delete_manifest() to write delete manifests with
  ManifestContentType::Deletes
- Update manifest_file() to include delete manifests
- Update summary() to populate delete file metrics
- Enhance validate_duplicate_files() for both data and delete files
- Add comprehensive unit tests

This lays the groundwork for operations like RowDelta that need to
atomically commit both data files and delete files.

Author: ethan-tyler

crates/iceberg/src/transaction/append.rs

@@ -90,6 +90,7 @@ impl TransactionAction for FastAppendAction {
             self.key_metadata.clone(),
             self.snapshot_properties.clone(),
             self.added_data_files.clone(),
+            vec![],
         );
 
         // validate added files