[codex] fix generator function constructor lookups

[andrewtdiz]

Summary

• route early closure property reads for generator functions through the generator constructor/prototype helpers
• make g.constructor resolve via the %Generator% / %AsyncGenerator% prototype's constructor property, matching Object.getPrototypeOf(g).constructor

Root Cause

js_object_get_field_by_name had an early is_closure_ptr fast path that returned undefined for constructor/prototype before the later GC_TYPE_CLOSURE branch could apply the existing generator intrinsic handling. That made g.constructor.name fail even though the generator intrinsic tower and Object.getPrototypeOf(g).constructor were present.

Validation

• cargo check -p perry-runtime
• PERRY_NO_AUTO_OPTIMIZE=1 npm exec --yes --package=node@26 -- bash -lc 'node --version; ./run_parity_tests.sh --suite node-suite --module globals --filter generator-intrinsics'
• PERRY_NO_AUTO_OPTIMIZE=1 npm exec --yes --package=node@26 -- bash -lc 'node --version; ./run_parity_tests.sh --suite node-suite --module globals --filter generator-prototype'
• PERRY_NO_AUTO_OPTIMIZE=1 npm exec --yes --package=node@26 -- bash -lc 'node --version; ./run_parity_tests.sh --suite node-suite --module util --filter generator-predicates'
• cargo fmt --all -- --check
• git diff --check
• ./scripts/check_file_size.sh