Which issue does this PR close?

• Closes BUG: Parquet name mapping still uses position fallback for projection and predicate pushdown #2403.

What changes are included in this PR?

When a Parquet file lacks embedded field IDs but a name mapping (schema.name-mapping.default) is available, the reader applies the mapping to the Arrow schema — but it still planned column projection and predicate pushdown with the position-based fallback (field id N → column N-1). As a result:

• Projected columns whose mapped field IDs don't line up with their physical positions were silently read as all-NULL.
• Predicates were evaluated against the wrong physical columns, silently returning wrong rows (this also makes row-group pruning and row selection prune incorrectly).

This contradicts Java's ReadConf, which uses a three-branch strategy: embedded IDs → field-id projection; name mapping → apply mapping, then field-id projection; neither → position fallback. Our code applied the name mapping to the Arrow schema but then took the fallback branch anyway.

Changes:

• pipeline.rs: compute use_position_fallback = missing_field_ids && task.name_mapping.is_none() and pass it (instead of missing_field_ids) to both get_arrow_projection_mask and build_field_id_set_and_map, so position fallback only applies when there are no embedded field IDs and no name mapping.
• projection.rs: build_field_id_set_and_map now distinguishes the name-mapping case. When the Parquet descriptor has no embedded field IDs but a name mapping assigned IDs to the Arrow schema, it builds the field-id → leaf-column map from the Arrow schema's PARQUET:field_id metadata (new build_field_id_map_from_arrow_schema). Arrow leaf ordering matches Parquet leaf column ordering (both depth-first), the same invariant get_arrow_projection_mask already relies on for ProjectionMask::leaves.

Are these changes tested?

Two new regression tests in projection.rs, covering both broken paths:

• test_read_parquet_with_name_mapping_uses_mapped_field_ids: a file without field IDs whose columns [name, subdept] map to non-contiguous field IDs (2, 4). On main, projection NULL-fills both columns; with this fix, the values are read correctly.
• test_predicate_on_name_mapped_file_uses_mapped_field_ids: a predicate name = "Alice" on the same file shape, with row-group filtering and row selection enabled. On main, the predicate is evaluated against the wrong physical column and returns 2 rows; with this fix, it returns the single correct row.

Both tests fail on main (verified by reverting the pipeline.rs strategy line) and pass with the fix. The full arrow:: unit-test suite (97 tests) passes.