test: add comprehensive VFS module hooks coverage tests

Add 33 test cases targeting uncovered code paths in the new
self-contained module.registerHooks implementation: package exports
resolution (string, subpath, conditional, nested, default, array),
bare specifier resolution (main field, index fallback, extension
resolution, scoped packages), format detection (package type walk,
node_modules boundary, .cjs/.mjs/.json), directory entry resolution,
and error handling (invalid JSON, null exports).

Author: mcollina