fix lean-spec-tests-state-transition hive suite

[SoarinSkySagar]

Brings the state-transition spec-test suite to green against the devnet-5 fixtures.

Why it was failing

• Fixture schema drift: devnet-5 renamed the test field expect_exception → rejection_reason, and some fixtures now omit post.slot. Our deserializer still expected the old names and a mandatory slot, so those cases failed to parse.
• Missing edge-case invariants: new fixtures cover empty validator registries, empty aggregation bits, and attestations referencing slots beyond the chain view. The client either panicked (assert! on empty indices, out-of-bounds historical_block_hashes.get()) or failed to reject inputs the spec expects to be rejected.

How it's fixed

• Updated TestCase/PostState to the new field names and made post.slot optional; the runner now only checks the slot when a fixture provides it.
• Replaced panics in state transition with graceful ensure!/skip logic: reject proposer scheduling on an empty validator set, reject empty aggregation bits, and skip attestations whose source/target/head slot is past the chain view.
• Reworked the empty-blocks rejection probe in test_driver to build and run a header through process_block_header so the invariant actually fires and surfaces as succeeded: false.
• Realigned the fork-choice validator unit tests with the updated produce_block/build_block signatures.

[ArtiomTr]

Looks good, but:

1. Please fix merge conflicts
2. Squash your commits
3. Provide readable PR title + description. We don't use chore:/feat:/etc. prefixes, so you can just use plain title for that.

[SoarinSkySagar]

@ArtiomTr ready for review again

[ArtiomTr]

perfect 👍