test: verify changed-file scans skip unchanged data files

[osipovartem]

Summary

• Add a physical-plan assertion to the existing Iceberg v3 row-lineage/change-scan test.
• With three append snapshots, the full scan plans three data files while a change scan starting after the first snapshot plans only two. Existing row assertions confirm those two files contain the changed rows.
• This verifies the changed-data-file I/O criterion in Embucket/rustice#358 without changing runtime code.

Verification

• CARGO_INCREMENTAL=0 cargo +1.99.0 test -p datafusion_iceberg --lib test_v3_row_lineage_columns_are_synthesized_by_scan -- --nocapture (passed)
• cargo +1.99.0 fmt --all and git diff --check (passed)
• Independent read-only review: APPROVED.