chore: make post envelope beacon api spec compliant

[shane-moore]

Issue Addressed

Part of #8828 for the stateful path. Aligns produceBlockV4 and POST /eth/v1/beacon/execution_payload_envelopes with beacon-APIs #580 and #624.

Proposed Changes

• Request stateful block production with required include_payload=false
• Submit SignedExecutionPayloadEnvelope with required Eth-Consensus-Version and Eth-Blob-Data-Included: false headers
• Support broadcast_validation (gossip, consensus, consensus_and_equivocation)
• Return 202 when the envelope is broadcast but not fully imported

Additional Info

• Stateless SignedExecutionPayloadEnvelopeContents support is out of scope
• Stateful cache hardening is handled separately
• 415 on bad Content-Type is not addressed, matching the existing block POST flow

[eserilev]

LGTM

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-07-20 11:34 UTC · Rule: default · triggered by rule Add ready-to-merge labeled PRs to merge queue
• ✅ Checks passed · on draft merge queue: checking unstable (3b39ffe) and #9276 together #9667
• ✅ Merged — 2026-07-20 12:03 UTC · at fc145a1e9ad8f904f360a15a3143d6ecc0a0e015 · squash

This pull request spent 29 minutes 38 seconds in the queue, including 27 minutes 44 seconds running CI.

Required conditions to merge

• check-success=local-testnet-success
• check-success=test-suite-success
• github-review-approved [🛡 GitHub branch protection] (documentation)
  • chore: make post envelope beacon api spec compliant #9276