tests: align test vectors with proposal/attestation key split behavior

[unnawut]

🗒️ Description

After #449, the tests no longer gets a free proposer attestation included in each block. While the reftests were passing, they were passing because we introduced gossip_proposer_attestation and include_store_attestations that are opaque to the clients.

There were also some test cases where we assumed that a block always has an attestation, and so an additional fork depth guarantees an additional fork weight. But it's misleading to rely on fork depth because the fork choice rule doesn't consider depth at all.

This PR brings back the visibility by:

1. Explicitly including proposer's attestation in the next block
2. Remove gossip_proposer_attestation and include_store_attestations, so that when clients parse a BlockStep, it only needs to process it through store.on_block() and no other ops.
3. Rewrote tests that were relying on fork depth as fork weight

Also removed test_auto_collect_* test because those are testing behaviors only specific to having proposer_attestation

Some review notes: If you see non-obvious changes in this PR, they're from (3) above because fork depths are replaced with attestations. The rest are just turning the implicit proposer attestations into explicit block.attestations.

🔗 Related Issues or PRs

✅ 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 ./docs/ directory.

[tcoratger]

Thanks a lot!