[2026-08-29] Resolve generated relation subquery entity metadata

[philipgreat]

Gap

Generated relation predicates pass the child entity name into in_subquery and not_in_subquery, while SQL compilation currently expects an EntityDescriptor. Source generation succeeds but SQLite execution cannot resolve the nested entity metadata.

Acceptance

• Resolve nested subquery entity names through the installed schema provider before SQL compilation.
• Preserve explicit descriptors for hand-built expressions.
• Verify positive and negative relation subqueries against SQLite.