test(fc): produce block enforces MAX_ATTESTATIONS_DATA limit

[tcoratger]

Context

Each block can include at most `MAX_ATTESTATIONS_DATA = 16` distinct `AttestationData` entries. When the store has more than 16 pending attestation data items, the block builder must select a subset using greedy selection (maximizing validator coverage).

This limit exists to bound block size and processing time. Client teams must enforce it.

No spec test filler currently tests the MAX_ATTESTATIONS_DATA boundary.

What to test

Write a fork choice filler that:

1. Uses enough validators (e.g., 20+) to create many distinct attestation data entries
2. Gossips attestations with more than 16 different `AttestationData` (different target slots, different target roots via forks, etc.)
3. Produces a block
4. Verifies the block contains exactly 16 `AttestationData` entries (not more)

Key assertions

• `StoreChecks(block_attestation_count=16)` — exactly at the limit
• No error from the block builder
• The 16 selected entries should maximize validator coverage (greedy selection)

Where to add the test

Add to: `tests/consensus/devnet/fc/test_block_production.py`

Code skeleton

def test_produce_block_enforces_max_attestations_data_limit(
    fork_choice_test: ForkChoiceTestFiller,
) -> None:
    """Block production caps attestation data entries at MAX_ATTESTATIONS_DATA (16)."""
    # Strategy: create a chain with many forks to generate >16 distinct
    # AttestationData entries (each targeting a different root/slot combo).
    #
    # With 20 validators and multiple forks, we can create many distinct
    # target (slot, root) pairs. Then verify the block only includes 16.
    #
    # TODO: design the fork/attestation topology to exceed 16 distinct entries
    pass

Note: Creating 16+ distinct `AttestationData` entries requires careful fork topology. Each unique (source, target, head) triple is a distinct entry. Multiple forks at different slots is the simplest approach.

How to run

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

References

• `MAX_ATTESTATIONS_DATA = 16`: `src/lean_spec/subspecs/chain/config.py`
• `State.build_block`: `src/lean_spec/subspecs/containers/state/state.py` — enforces the limit
• `select_greedily`: `src/lean_spec/subspecs/xmss/aggregation.py` — greedy selection algorithm

[0xAysh]

I’l like to work on this

[shaaibu7]

I'd like to handle this

[tcoratger]

@0xAysh Assigned