Add tests for EIP 7251.

[bshastry]

Issue Addressed

This PR addresses the testing coverage for EIP-7251's partial withdrawal functionality in Lighthouse, specifically the is_partially_withdrawable_validator_electra implementation. No existing issue number, but this adds test coverage for the Electra fork implementation.

Proposed Changes

1. Adds test test_partial_withdrawals_electra in consensus/types/src/validator.rs that covers four scenarios:

   • Compounding validator (0x02 prefix) with 2048 ETH effective balance and excess balance
   • Non-compounding validator (0x01 prefix) with 2048 ETH effective balance
   • Compounding validator with 32 ETH effective balance
   • Non-compounding validator with 32 ETH effective balance

2. Test verifies that automatic partial withdrawals are only allowed when:

   • Validator has the correct effective balance for both Eth1 and compounding modes
   • Validator has excess balance above effective balance

Additional Info

1. Test Coverage: This test specifically covers the automatic sweep mechanism for excess balances, which is distinct from user-initiated partial withdrawals.

2. Future Considerations:

   • May want to add additional test cases for edge cases around the effective balance values
   • Could extend to test interaction with different fork transitions
   • Consider adding integration tests with actual withdrawal processing

3. Related Specs:

   • EIP-7251
   • Electra Consensus Spec

4. Review Notes:

   • Test uses both 0x01 and 0x02 withdrawal credential prefixes
   • Balance values are chosen to test excess balance scenarios
   • All assertions include descriptive messages for failure reporting