fix: expose function return tables outside search path

[aqilaziz]

What changed

• Includes tables/views referenced by functions on the active search_path in the loaded SQL context, even when the referenced table lives in another schema with USAGE privilege.
• Keeps non-search-path referenced tables out of root collection and mutation fields while still allowing typed function return values.
• Adds regression coverage for a function in a2 returning a table row type from a1.

Fixes #455

Verification

• cargo fmt --check
• git diff --cached --check

cargo check could not complete in this local environment because pgrx requires $PGRX_HOME and it is not initialized here.