feat: support concurrent chunk uploads

[TorstenDittmann]

This PR updates the SDK to support concurrent chunk uploads.

[greptile-apps]

Greptile Summary

This PR refactors FileUpload to upload multi-chunk files with up to 8 concurrent workers and bumps the SDK version to v4.1.0. Chunk 0 is always uploaded synchronously to retrieve the server-assigned upload ID before the concurrent fan-out begins, and a done channel is used to signal workers to stop on the first error.

• copyHeaders/copyParams helpers ensure each worker operates on its own per-request copies, preventing concurrent mutation.
• A lastResult fallback is added so a non-nil final response is always returned even when no single response reports full completion.
• Previously flagged concerns (no early cancellation, stale result, parseUploadId outer-err capture) are all addressed in this revision.

Confidence Score: 5/5

The concurrent upload implementation is safe to merge: goroutine lifecycle is correctly managed, the upload ID is fully resolved before any worker starts, and per-chunk copies of headers and params prevent concurrent mutation.

The core concurrency concerns are all handled correctly — done-channel cancellation fires at most once, workers can always drain into a fully-buffered results channel without blocking, os.File.ReadAt is safe for concurrent use via pread, and uploadId is written before goroutines start and only read afterward. The one new finding is a naming-collision style nit with no runtime impact.

No files require special attention; client/client.go is the only changed file and the concurrent logic is sound.

Important Files Changed

Filename	Overview
client/client.go	Refactors multi-chunk upload to use up to 8 concurrent workers with a jobs/results/done channel pattern; uploads chunk 0 synchronously to capture the upload ID, then fans out the remainder. Previously flagged issues have been addressed. One style issue: the added result = resp line sits immediately above a shadowing inner var result map[string]interface{} declaration.

_{Reviews (2): Last reviewed commit: "feat: support concurrent chunk uploads" | Re-trigger Greptile}