fix(writer): validate equality delete field ids

[u70b3]

Which issue does this PR close?

• Closes Equality delete writer should reject invalid equality field IDs #2722.

What changes are included in this PR?

• Add writer config validation for equality delete field IDs.
• Reject empty equality ID lists, duplicate equality field IDs, and field IDs that are missing from the table schema.
• Keep the existing RecordBatchProjector behavior for unsupported field types and map/list reachability.
• Fix a stale comment: equality delete fields may be optional, unlike identifier fields.

Are these changes tested?

• cargo test -p iceberg test_equality_delete_rejects_empty_equality_ids --lib
• cargo test -p iceberg test_equality_delete_rejects_duplicate_equality_ids --lib
• cargo test -p iceberg test_equality_delete_rejects_missing_equality_id --lib
• cargo test -p iceberg test_equality_delete_unreachable_column --lib
• cargo test -p iceberg test_equality_delete_with_nullable_field --lib
• cargo test -p iceberg equality_delete --lib
• cargo fmt --check
• cargo test -p iceberg --lib
• cargo clippy -p iceberg --all-features --lib --tests -- -D warnings
• make check
• make unit-test
• Docker-backed integration path:
  • docker compose -f dev/docker-compose.yaml up -d --build --wait
  • make nextest (1832 passed, 0 skipped)

[viirya]

Confirmed the gap: EqualityDeleteWriterConfig::new on main goes straight to schema_to_arrow_schema + RecordBatchProjector::new with no validation of equality_ids, so an empty / duplicated / non-existent id was silently accepted. Adding these checks is a real improvement.

The comment fix is accurate against the spec — equality-delete fields do follow the identifier-field type restrictions except that optional columns (and columns under optional structs) are allowed. I checked the spec wording and it matches what you wrote.

One thing worth flagging for the record: the duplicate-id and "id must exist in schema" checks go a bit beyond the Java reference — Java core guards against null/empty (GenericAppenderFactory, FileWriterBuilderImpl) but does not check duplicates or schema membership. I think adding them is the right call (a duplicate equality id is a clear user error, and an id absent from the schema would produce a meaningless delete file), just noting it's stricter than Java so it's a deliberate choice rather than an oversight.

Pulled the branch: all 11 equality_delete tests pass (including the 5 new ones), clippy and rustfmt clean. LGTM.

[u70b3]

@viirya Thanks for the thorough and rigorous review — really appreciate you cross-checking the spec wording and the Java reference implementation.

You're right that the duplicate-id and schema-membership checks are deliberate stricter guards. A duplicate equality id is almost certainly a user/configuration error, and an id missing from the schema would lead to a confusing failure later (or a meaningless delete file). Failing fast here with a clear error message feels like the right behavior for the Rust writer.

I'll also open a corresponding PR against the Java SDK to add the same duplicate / missing-id validation there, so the two implementations stay aligned on this stricter behavior. I'll link it back to this PR once it's up.

Thanks again for the LGTM!