[RaphaelFakhri]

Summary

track_batch and import_batch split the events into request batches when max_concurrent_requests is set, or when max_batch_size is greater than 50. Each request batch reported its errors to the next step, but only the last request batch's errors reached the user callback. A failure in an earlier request batch was dropped, and the callback could receive null even though events were rejected.

This change collects the errors from every request batch and passes the combined list to the callback. The callback still receives null when every request succeeds, so the existing contract is unchanged.

The results argument still comes from the last request batch, as before. Its entries are always undefined, so this change leaves it alone to keep the diff small.

Testing

• Added passes errors from every request batch to callback to test/track.js (max_concurrent_requests: 1) and test/import.js (max_batch_size: 100). In each test the first request fails and the rest succeed.
• Without the fix, both tests fail because the callback receives null. With the fix, both pass.
• npm test: 489 + 56 tests pass. npm run lint and npm run check-format are clean (the one existing lint warning is in the openfeature package tests).

Fixes #295