fix(pnp): provide source for commonjs files inside zip archives

[tellnes]

What's the problem this PR addresses?

The PnP ESM loader fails with EBADF: bad file descriptor, fstat on Node.js >= 25.7.0 when ESM code imports a CommonJS package from a zip archive.

Bisected results:

Node version	Status
24.14.1	Works
25.0.0	Works
25.4.0	Works
25.6.0	Works
25.6.1	Works
25.7.0	Broken
25.8.1	Broken

The regression was introduced by nodejs/node#61769 ("lib: reduce cycles in esm loader and load it in snapshot"), which changed how createCJSModuleWrap handles null source. When the PnP loader returns undefined source for CJS modules, Node's fallback now calls getSourceSync() which tries to readFileSync the zip-internal path and fails.

Error:

node:fs:386
  const stats = binding.fstat(fd, false, undefined, true /* shouldNotThrow */);
                        ^

Error: EBADF: bad file descriptor, fstat
    at tryStatSync (node:fs:386:25)
    at readFileSync (node:fs:442:17)
    at getSourceSync (node:internal/modules/esm/load:37:14)
    at createCJSModuleWrap (node:internal/modules/esm/translators:204:32)

How did you fix it?

For CommonJS files inside zip archives (.zip/ in path), the PnP load hook now returns the source content instead of undefined. This prevents Node from trying to read the file from disk.

For CJS files on the real filesystem, undefined is still returned to preserve the CJS monkey-patching behavior (cf. #5677).

Checklist

• I have read the Contributing Guide.
• I have set the packages that need to be released for my changes to be effective.
• I will check that all automated PR checks pass before the PR gets reviewed.