Deduplicate LFS transfer test setup

What does this MR do and why?

Problem

lfstransfer_test.go repeats LFS payload, header, signing, and batch response parsing logic across several scenarios.

Solution

• Extract helpers for standard headers and recurring LFS ID payloads.
• Centralize ID encoding and HMAC-SHA256 token generation.
• Centralize batch item parsing and token verification.
• Reuse shared test secrets and handler headers.

No change in behavior.

References

How to set up and validate locally

1. Run go test ./internal/command/lfstransfer/....
2. Run go vet.
3. Run gofmt -d.

MR acceptance checklist

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