test: add BuildFlavorVersion unit tests

[ktrubilo9]

Summary

Adds unit tests for BuildFlavorVersion.
The test verify that:

• file hashes are computed from the uploaded changeset and passed to the repository,
• required repository calls are executed,
• missing uploaded files return ErrFlavorFilesNotUploaded,
• hash computation is skipped when files are already marked as uploaded.

Also adds a small test helper for creating in-memory tar.gz changeset used by the hash computation path.

Testing

go test ./controlplane/chunk -run TestBuildFlavorVersion -count=1 -v

Closes #487

[freggy]

thank you very much for your contribution @ktrubilo9! :) I reviewed your PR and left some comments.

[ktrubilo9]

Thanks for review, I've addressed both comments, replaced the helper with test.CreateTarGz and added comparisions againts xxh3 hashes of the file contents. Also fixed the lint issue.

[freggy]

great, thanks! i'll merge once ci passes