IcebergMetadataTableProvider ignores `projection` in `scan()`: projected/aggregate queries on populated metadata tables fail

[gmhelmold]

Apache Iceberg Rust version

main (d1b8eac)

Describe the bug

The datafusion integration's metadata table provider ignores the projection argument in TableProvider::scan() (crates/integrations/datafusion/src/table/metadata_table.rs): it always returns an IcebergMetadataScan that reports the full metadata-table schema and emits unprojected batches. (limit is also unused, but that's only a missed optimization; filters never reach scan() since supports_filters_pushdown isn't overridden — projection is the correctness issue.)

This goes unnoticed because existing tests only query empty metadata tables with SELECT * (test_metadata_table in integration_datafusion_test.rs; the $snapshots checks in insert_into.slt run before any snapshot exists). Same class as #828, which fixed this for IcebergTableScan.

To Reproduce

Any populated metadata table — e.g. against the seeded table in the crates/sqllogictest harness:

INSERT INTO default.default.test_partitioned_table VALUES (1, 'a', 'b');

SELECT count(*) FROM default.default.test_partitioned_table$snapshots;
-- DataFusion error: Internal error: Physical input schema should be the same as the
-- one converted from logical input schema. Differences:
--   - Different number of fields: (physical) 6 vs (logical) 0.

SELECT operation FROM default.default.test_partitioned_table$snapshots;
-- returns all 6 snapshot columns instead of just `operation`

Expected behavior

scan() honors projection: make IcebergMetadataScan projection-aware (report the projected schema in its plan properties and project the emitted batches), mirroring how IcebergTableScan handles projection.

Willingness to contribute

Yes — happy to submit the fix with sqllogictest regression coverage on populated metadata tables.