[codex] fix re-exported class method prefixes

[andrewtdiz]

Summary

• Canonicalize ImportedClass.source_prefix through the class defining path for namespace imports, namespace re-exports, and named imports.
• Keep existing barrel/effective prefixes for non-class import metadata while using the defining module prefix for class method symbols.
• Add an RxJS-style barrel regression where Subject imports Observable via the package index and inherited methods are exercised.

Root Cause

exported_classes can map barrel names to class objects emitted from internal modules. The importer was storing the barrel/effective module prefix in ImportedClass.source_prefix, so generated call sites could reference method symbols with the barrel prefix instead of the module that emitted those methods.

Validation

• cargo build --release --quiet -p perry -p perry-runtime -p perry-stdlib
• cargo test --release -p perry canonical_class_source_prefix_prefers_defining_path -- --nocapture
• ./target/release/perry test-files/test_issue_1021_rxjs_reexport_methods.ts -o /tmp/perry_issue_1021_reexport_methods_after && /tmp/perry_issue_1021_reexport_methods_after
• cargo fmt --all -- --check
• git diff --check
• ./scripts/check_file_size.sh

Notes

I also tried the current tests/release/packages/nestjs-hello fixture with this branch. It now stops before link/codegen at the V8-free JS dependency gate for Express/Multer-side dependencies such as tslib, uid, iterare, fast-safe-stringify, path-to-regexp, cors, express, and multer, so this PR does not claim an end-to-end NestJS pass.