fix: validate substrait filters that need no field pruning

[1fanwang]

Which issue does this PR close?

• Closes Substrait filter validation (lambda/window/subquery rejection) is skipped when no schema fields are pruned #7805.

Rationale for this change

remap_expr_references doubles as the validation pass that rejects operators we cannot push down (window functions, subqueries, lambdas), but parse_substrait only calls it when remove_extension_types actually drops a field. A filter whose base schema has no extension types skips the checks entirely, and the unsupported expression reaches the DataFusion consumer instead.

The user sees a consumer-level error rather than ours. A subquery over an unpruned schema reports:

Substrait error: Subquery expression without SubqueryType is not allowed

instead of Window functions or subqueries not allowed in filter expression.

What changes are included in this PR?

remap_expr_references now runs unconditionally.

The guard it replaces is not load-bearing for the remapping itself: remove_extension_types builds the index mapping by walking every field and only advancing the destination counter for fields it keeps, so when nothing is dropped the mapping is the identity and the remap is a no-op. Dropping the guard adds the validation without changing any index.

This keeps validation and remapping as one traversal rather than two that have to be kept in step.

Are these changes tested?

Yes. test_unsupported_operator_rejected_without_pruning parses a subquery and a lambda against a single-Int32 base schema, which needs no pruning, and asserts each is rejected with its own message.

On the current code the subquery case reaches the consumer instead:

cargo test -p lance-datafusion --features substrait --lib substrait::tests::test_unsupported_operator

Before

running 1 test
test substrait::tests::test_unsupported_operator_rejected_without_pruning ... FAILED

---- substrait::tests::test_unsupported_operator_rejected_without_pruning stdout ----
thread '...' panicked at rust/lance-datafusion/src/substrait.rs:712:9:
unexpected error: LanceError(IO): Substrait error: Subquery expression without SubqueryType is not allowed, rust/lance-datafusion/src/substrait.rs:333:9

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 84 filtered out

After

running 14 tests
test substrait::tests::test_parse_substrait_aggregate_multiple_aggregates ... ok
test substrait::tests::test_parse_substrait_aggregate_sum ... ok
test substrait::tests::test_parse_substrait_aggregate_count_star ... ok
test substrait::tests::test_parse_substrait_aggregate_sum_with_group_by ... ok
test substrait::tests::test_substrait_conversion ... ok
test substrait::tests::test_expr_substrait_roundtrip ... ok
test substrait::tests::test_substrait_roundtrip_with_list_struct_struct ... ok
test substrait::tests::test_substrait_roundtrip_with_fixed_size_list_column ... ok
test substrait::tests::test_substrait_roundtrip_like ... ok
test substrait::tests::test_substrait_roundtrip_with_list_of_struct ... ok
test substrait::tests::test_substrait_roundtrip_with_many_nested_columns ... ok
test substrait::tests::test_substrait_roundtrip_starts_with ... ok
test substrait::tests::test_unsupported_operator_rejected_without_pruning ... ok
test substrait::tests::test_substrait_roundtrip_with_null_and_float16_columns ... ok

test result: ok. 14 passed; 0 failed; 0 ignored; 0 measured; 71 filtered out

The roundtrip tests above cover the pruning and nested-schema paths, including List<Struct> and extension-type removal, so they would catch a remap regression. The whole lance-datafusion suite passes (85 tests).

Are there any user-facing changes?

Unsupported filter operators are now rejected with the descriptive error in the case that previously slipped through. No API change.

[lance-gatekeeper]

✅ Gate recommendation: approve.

The conditional-validation defect and its panic regressions are now covered: unsupported operators are checked for unpruned schemas, omitted roots remain compatible, and malformed non-leading IfThen clauses return an input error.