fix(reader): keep rows with a null equality-delete column value - #2781

Which issue does this PR close?

• Closes Equality delete on a nullable column deletes rows whose value in that column is null #2780.

What changes are included in this PR?

Equality deletes are applied by building a predicate that keeps rows not matched by any delete row. For each equality column the keep term was col != v, produced by negating the equality match. Applied as a row filter, null != v evaluates to null, so a data row whose equality column is null was dropped. Per the spec (Equality Delete Files), a null value matches only a null delete value, so such rows should be kept.

This builds the keep predicate directly in parse_equality_deletes_record_batch_stream: col IS NULL OR col != v for a non-null delete value, and col IS NOT NULL for a null delete value. Rows with a null value in an equality column are no longer deleted.

Are these changes tested?

Yes, unit tests in caching_delete_file_loader covering the predicate construction:

• test_equality_delete_predicate_preserves_null_rows: a non-null delete value on a nullable column produces (col IS NULL) OR (col != v).
• test_equality_delete_predicate_matches_null_delete_value: a null delete value produces col IS NOT NULL.
• test_equality_delete_predicate_multiple_columns: per-column keep terms are OR-ed.
• test_equality_delete_predicate_multiple_delete_rows: per-row keep predicates are AND-ed.
• test_delete_file_loader_parse_equality_deletes (existing, updated): the combined case with multiple columns, a required column, and null delete values.