add CI safe max validator genesis transition test

[shrirajpawar4]

🗒️ Description

Adds focused coverage for the max validator registry state-transition path from #580.

This test builds a genesis state directly with VALIDATOR_REGISTRY_LIMIT placeholder validators using zero pubkeys, instead of calling generate_pre_state(num_validators=4096). That avoids
generating 4096 XMSS keys or adding key fixtures.

It then processes a normal slot-1 block and uses one forced_attestations entry with validators 0..2730 to exercise the 2731-of-4096 justification tally in the STF. This is intentionally registry-
limit/state-transition coverage, not signature-generation coverage.

🔗 Related Issues or PRs

Closes #580

Related: #641

✅ Checklist

• Ran tox checks to avoid unnecessary CI fails:

  uvx tox
  

• Considered adding appropriate tests for the changes.
• Considered updating the online docs in the //leanEthereum/leanSpec/tree/main/docs/ directory.

Local verification

 uv run ruff check tests/consensus/devnet/state_transition/test_genesis.py

  uv run fill --fork=Lstar --output /tmp/leanSpec-issue580-fixtures --clean -k test_genesis_maximum_validators_with_forced_threshold_attestation

  The targeted fixture passed locally:

  1 passed, 553 deselected in 0.49s

[shrirajpawar4]

@tcoratger can you take a look please