Add execution proof observability

[frisitano]

Summary

• add bounded metrics for execution-proof receipt, verification, proof-engine latency, cache state, and payload availability
• add structured debug logging for gossip/HTTP receipt and proof-triggered payload availability
• expose pending-payload-cache proof-gating state and cover label/cache behavior with tests

Metrics

• pending_payload_cache_execution_proofs
• pending_payload_cache_payloads_awaiting_execution_proofs
• pending_payload_cache_required_execution_proofs
• beacon_execution_proof_received_total{source,proof_type}
• beacon_execution_proof_verification_total{proof_type,outcome,reason}
• beacon_execution_proof_engine_verification_seconds{proof_type,outcome}
• beacon_execution_proof_availability_total{outcome}

The proof-engine latency histogram follows Prometheus conventions by exporting seconds, with 1/2/5 ms-resolution buckets. The dashboard follow-up renders the values in milliseconds.

Testing

• cargo fmt --all -- --check
• cargo check -p beacon_chain -p network -p http_api
• cargo test -p beacon_chain --lib execution_proof
• cargo test --release -p http_api --test bn_http_api_tests execution_proof_tests::
• cargo clippy -p beacon_chain -p network -p http_api --all-targets -- -D warnings -A clippy::enum_variant_names -A clippy::question_mark

The two Clippy allowances cover existing baseline findings outside this change.

Follow-up

• matching ethereum-package Grafana dashboard updates are prepared locally and will be proposed separately
• image and devnet validation remain pending