fix(spec): check optional ancestors when binding IS NULL on a nested required field

[LuciferYang]

What

Predicate::bind rewrites IS NULL on a required field to AlwaysFalse (and IS NOT NULL to AlwaysTrue) from the leaf field's own required flag, without checking whether an ancestor struct is optional. The spec says a null parent struct implies its leaves are null, so the shortcut is only valid when the field and every ancestor are required.

With person optional and person.age required, person.age IS NULL bound to AlwaysFalse, so a scan dropped the rows whose person is null instead of keeping them. IS NOT NULL had the mirror bug: it bound to AlwaysTrue and kept those rows.

Closes #3292.

How

Added Schema::is_field_and_ancestors_required(field_id), which walks the parent chain and returns true only when the leaf and every ancestor struct is required. The parent chain comes from the existing index_parents, now computed once in build() and stored on Schema; validate_identifier_ids takes that shared map instead of recomputing its own. The two Unary arms in Predicate::bind use this check in place of the leaf-only required flag.

Tests

• test_bind_is_null_required_leaf_under_optional_parent_is_kept and its is_not_null sibling: a required leaf under an optional parent keeps the predicate instead of folding to a constant.
• test_bind_null_predicates_on_leaf_under_required_parent_short_circuit: a fully required path still folds to False / True.
• test_bind_is_null_required_leaf_under_optional_grandparent_is_kept: an optional struct two levels up is caught by the ancestor walk.
• test_is_field_and_ancestors_required: helper coverage for top-level fields and an unknown id.

End-to-end reads of a null parent through the Arrow row filter depend on #2906 (nested-leaf predicate pushdown); this PR fixes the binding layer, which is reachable and testable on its own.

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[JeonDaehong]

Checked this end to end with #2906 and #3453: an equality delete s.f = 20 on a required f under an optional s also removed the row whose s is null, because the delete's keep predicate s.f IS NULL OR s.f != 20 lost its IS NULL at bind time. With this PR that row is kept, as the spec says. cargo test -p iceberg --lib passes on current main with this merged. LGTM.