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)

Note: the Docker-backed run used local mirror substitutions for image and Python package downloads because direct pulls from Docker Hub timed out in my environment. No compose-file changes are included in this PR.