Set parent payload availability bit for attestation packing and the attestation simulator

[eserilev]

Payload processing is deferred to the next block, so a state from before process_parent_execution_payload doesn't have the parent's availability bit set.

This PR impls the following fixes:

• During attestation packing, set the parent's availability bit if the block builds on the parent's payload
• In the attestation simulator use the attested block's slot as parent_slot.

Only non same slot attestations read the availability bit, so these fixes are only relevant for attestations made on a skipped slot.

Depends on #9764

[mergify]

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Some required checks have failed. Could you please take a look @eserilev? 🙏

[mergify]

Some required checks have failed. Could you please take a look @eserilev? 🙏

[mergify]

Queued — the merge queue status continues in this comment ↓.

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-08-25 10:44 UTC · Rule: default · triggered by rule Add ready-to-merge labeled PRs to merge queue
• ✅ Checks passed · on draft merge queue: checking unstable (251e12c) and #9828 together #9915
• ✅ Merged — 2026-08-25 11:14 UTC · at 678d1dec6cd7b325dc1c21ff4223bf7904fc71d2 · squash

This pull request spent 29 minutes 45 seconds in the queue, including 27 minutes 21 seconds running CI.

Required conditions to merge

• check-success=local-testnet-success
• check-success=test-suite-success
• github-review-approved [🛡 GitHub branch protection]
  • Set parent payload availability bit for attestation packing and the attestation simulator #9828