test(st): block at very large slot number with many skipped slots

[tcoratger]

Context

When a block arrives at a slot far in the future (e.g., slot 500), the state transition must call `process_slots` to advance through all intermediate empty slots. For each skipped slot:

• `historical_block_hashes` is extended with `ZERO_HASH` entries
• `justified_slots` is extended with `False` entries
• The state root may be cached in the block header

This is a stress test for client implementations. A naive implementation might be slow or run out of memory with hundreds of empty slot transitions.

The existing filler `test_block_at_large_slot_number` jumps to slot 100. This test should push further (e.g., slot 500 or 1000) to stress-test the slot processing loop.

What to test

Write a state transition filler that:

1. Starts from genesis (slot 0)
2. Processes a block at a very large slot (e.g., slot 500)
3. Verifies the post-state has correct slot, historical block hashes count, and justified slots length

Key assertions

• `StateExpectation(slot=Slot(500))`
• `historical_block_hashes_count` should be 1 (only genesis parent) plus ZERO_HASH entries for all skipped slots
• `justified_slots` should be extended to cover all slots up to 499
• No errors or timeouts during processing

Where to add the test

Add to: `tests/consensus/devnet/state_transition/test_block_processing.py`

Code skeleton

def test_block_at_very_large_slot_with_many_skipped(
    state_transition_test: StateTransitionTestFiller,
) -> None:
    """State transition handles hundreds of empty slots without error."""
    state_transition_test(
        pre=generate_pre_state(num_validators=4, genesis_time=Uint64(0)),
        blocks=[BlockSpec(slot=Slot(500))],
        post=StateExpectation(
            slot=Slot(500),
            # historical_block_hashes should have entries for each slot:
            # genesis parent_root + ZERO_HASH * (500 - 1) skipped slots
            # The first block adds parent_root; skipped slots add ZERO_HASH
        ),
    )

How to run

uv run fill --fork=devnet --clean -n auto -k test_block_at_very_large_slot_with_many

References

• `State.process_slots`: `src/lean_spec/subspecs/containers/state/state.py`
• `State.process_block_header`: same file — extends `historical_block_hashes` and `justified_slots`
• Existing large slot test: `tests/consensus/devnet/state_transition/test_block_processing.py::test_block_at_large_slot_number`

[Smartdevs17]

I'd like to work on this issue. Will start on it shortly.

[shaaibu7]

I'd like to handle this