Fix block upload worker leaks after an error

[ClaudiuSchuster]

Problem

uploadPendingBlocks returned as soon as it received the first block-upload error. The remaining goroutines could then block while sending to the unbuffered result channel, still holding their weighted-semaphore slots. Repeated retries progressively consumed all block-upload slots and left later uploads unable to start.

The acquisition-error path also continued to a deferred Release even though no slot had been acquired.

Fix

• Buffer one result per block.
• Receive every worker result before returning the first error.
• Return immediately when acquiring a slot fails.
• Add a regression test that repeats failing batches and then acquires the full semaphore capacity.

Verification

Validated with Go 1.26 and golangci-lint 2.9.0, matching the repository workflow:

golangci-lint run --timeout=180s
go test -v ./...
go test -v -race ./...
go test -race -run '^TestCollectUploadErrorsReleasesAllWorkersAfterFailure$' -count=100 ./...
go build ./...

All commands pass. The repository's credential-gated Proton integration tests are skipped by their existing guard; all unit tests, including the new regression test, run successfully.

The fork's repository workflow also passes on this exact commit: Lint and Test.

[ClaudiuSchuster]

Additional real-world validation from the downstream PDrive integration:

• the exact worker-drain change was deployed in a checksum-pinned rclone build during one 30.4 GiB VFS upload;
• six separated, same-process Proton block-upload 502 cycles occurred after that build became active;
• the same rclone process continued without a systemd restart and payload progress resumed after the failures;
• the upload has now completed with an empty queue and VFS cache.

This supports the semaphore-release behavior under repeated real backend failures, beyond the unit and race tests already listed in the PR.

A separate bounded retry for only the failed encrypted blocks is tracked in oss-singularity/proton-drive-linux#42 and was validated in oss-singularity/Proton-API-Bridge#2. I am deliberately keeping that follow-up out of this PR so this worker-lifecycle fix remains focused. Once this prerequisite lands, the retry can be proposed upstream as a single independent commit.