Refactor: Improve WorkerError Handling and Stack Trace Merging for Thread Loader

[Manas-Dikshit]

This pull request refactors the WorkerError implementation to make it more reliable and easier to read.
The new version handles edge cases more safely, avoids runtime errors, and produces clearer stack traces for worker threads.

What Changed
Replaced the old stack() helper with a safer buildStackTrace() utility
Added default values for missing properties (err, err.stack, workerId)
Improved how stack trace differences are calculated and merged
Cleaned up variable names and improved code readability
Ensured consistent formatting across all generated error messages

Why This Matters
The previous version could throw additional errors if err or its stack were missing.
With these improvements, developers get cleaner output and more reliable debugging information when using thread-based loaders.

Example Output
Thread Loader (Worker 3)
ReferenceError: x is not defined
at doSomething (worker.js:25:11)
at Worker.run (thread.js:10:5)

Next Steps
Consider adding a toJSON() method to support structured logging or error reporting across worker boundaries.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: Manas-Dikshit / name: Manas Ranjan Dikshit (04f58c8, 091f354, 39900f2, 47f7f85, b584f1a)

[alexander-akait]

Please add a test case

[Manas-Dikshit]

Thanks for pointing that out, Alexander – I’ve added a specific test case for the new WorkerError behavior.
It includes tests for correct stack merging, message handling, and default fallbacks when err or workerId are absent.

[alexander-akait]

Please add integration test, not unit tests, they are useles

[alexander-akait]

Please use our test infrastructure, not own

[alexander-akait]

Please use EXISTING infrastructure - https://github.com/webpack/thread-loader/blob/main/test/webpack.test.js