fix: handle undefined __filename in bundled environments

[mcollina]

Summary

When undici is bundled inside Node.js as an internal module, __filename is not defined. This causes a ReferenceError when fetch() fails, which swallows the original error and replaces it with the ReferenceError, making err.cause undefined.

This was introduced in #4778.

The Problem

The code added in #4778:

module.exports.fetch = function fetch (init, options = undefined) {
  return fetchImpl(init, options).catch(err => {
    appendFetchStackTrace(err, __filename)  // __filename is undefined!
    throw err
  })
}

When bundled in Node.js internals, __filename is not available, causing:

ReferenceError: __filename is not defined

This error is thrown inside the .catch() handler, replacing the original fetch error. As a result, err.cause becomes undefined instead of containing the actual error.

The Fix

1. Safely capture __filename at module load time, defaulting to undefined if not available
2. Use the new stack trace augmentation when __filename is available
3. Fall back to the original Error.captureStackTrace(err) behavior when __filename is not available

Testing

• Verified undici's stack trace tests still pass: npx borp -p "test/fetch/client-error-stack-trace.js"
• Verified Node.js tests pass with the fix:
  • test-permission-net-fetch.js
  • test-tls-set-default-ca-certificates-append-fetch.mjs
  • test-tls-set-default-ca-certificates-reset-fetch.mjs

Refs: nodejs/node#61683, nodejs/node#61679

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.27%. Comparing base (9cc025b) to head (cbe533b).

Files with missing lines	Patch %	Lines
index.js	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4812      +/-   ##
==========================================
+ Coverage   93.26%   93.27%   +0.01%     
==========================================
  Files         109      109              
  Lines       34098    34106       +8     
==========================================
+ Hits        31801    31812      +11     
+ Misses       2297     2294       -3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.