fix: subpath exports resolution for CJS files in dual-format packages

[RizonSunny]

Packages like @langchain/langgraph use the "exports" field to map subpaths (e.g., ./prebuilt) to .cjs files. The resolver correctly found these files but discarded the result because it gated on isESM — which is false for .cjs files. This caused a fallback to the legacy resolver that doesn't understand exports, resulting in MODULE_NOT_FOUND errors.

Changes:

• resolver.ts: Add resolvedViaExports flag to distinguish "resolved
  via exports map" from "file is ESM"
• follow.ts: Gate on resolvedViaExports instead of isESM so CJS
  files resolved through exports are accepted
• walker.ts: Handle nested condition objects (e.g., require.default)
  in synthetic main inference

Testing library:

• @langchain/langgraph": "^1.0.15"
• @hyzyla/pdfium": "^2.1.9

[robertsLando]

Thanks for tackling this — the core idea (using resolvedViaExports instead of isESM to gate exports-aware resolution) is the right fix. However there are a few issues that need to be addressed before merging.

1. rewriteMjsRequirePaths removal — this will break .mjs imports

The removal of rewriteMjsRequirePaths in walker.ts is unrelated to the CJS subpath exports fix and is unsafe. The call chain it supports is:

1. ESM .mjs files are transformed to CJS by esbuild → import './foo.mjs' becomes require('./foo.mjs')
2. The packer renames .mjs → .js in the snapshot (packer.ts:84-86)
3. rewriteMjsRequirePaths rewrites require('./foo.mjs') → require('./foo.js') to match

Without step 3, transformed .mjs files with relative imports to other .mjs files will have dangling require() paths at runtime. Please keep this code.

2. Deduplicate the extensions array

The extensions list ['.js', '.json', '.node', '.mjs', '.cjs'] is now duplicated across three files (walker.ts, follow.ts, resolver.ts). Please export MODULE_RESOLVE_EXTENSIONS from a shared location (e.g. common.ts) and import it in all three places instead of maintaining three independent copies.

3. PR description mentions missing feature

The description says "Handle nested condition objects (e.g., require.default) in synthetic main inference" in walker.ts, but there's no such change in the diff. Please either add the implementation or update the description.

4. Please add a test

This fix needs a test case that covers subpath exports resolution for CJS files in dual-format packages (e.g., a package with an exports field that maps a subpath to a .cjs file). This will prevent regressions and validate the resolvedViaExports flag works as intended.

[robertsLando]

Please keep the rewriteMjsRequirePaths import and usage — it's needed for .mjs files that get transformed to CJS. The packer renames .mjs → .js in the snapshot, and without this rewrite, require('./foo.mjs') paths generated by esbuild will point to files that no longer exist.

[robertsLando]

This extensions array is now duplicated in three places (walker.ts:80, follow.ts:76, resolver.ts:152). Please extract it to a shared constant (e.g. export MODULE_RESOLVE_EXTENSIONS from common.ts) and import it everywhere.

[robertsLando]

Same here — import the shared MODULE_RESOLVE_EXTENSIONS constant instead of hardcoding.