framework: export test fixtures as build artifacts

[unnawut]

For example when a new test fixture is added e.g. #195, it would be nice if the generated fixtures are viewable from the build job so that we can review the final output without having to clone and rerun uv fill locally for each PR changes. Right now we can only see that the fill passed, e.g. https://github.com/leanEthereum/leanSpec/actions/runs/19718792605/job/56497084126?pr=195

[KolbyML]

For example when a new test fixture is added e.g. #195, it would be nice if the generated fixtures are viewable from the build job so that we can review the final output without having to clone and rerun uv fill locally for each PR changes. Right now we can only see that the fill passed, e.g. https://github.com/leanEthereum/leanSpec/actions/runs/19718792605/job/56497084126?pr=195

Wouldn't this be a bad idea, because after a while it would just be printing spam?

[KolbyML]

Also shouldn't def test_genesis_block_hash_comparison() -> None: not be in tests/consensus/devnet/state_transition/test_genesis.py since that test doesn't generate test vectors it should be in tests/lean_spec/subspecs/xyz