fix(rpc): close signed-read replay gap via decoder-level rejection

[samlaf]

Depends on SeismicSystems/seismic-alloy#104.

Signed-read seismic transactions were previously rejected only at mempool admission. This might (?) work in a TEE world but is fragile, and given that we are planning to go to mainnet without TEEs it was a real issue. We might also one day want to enable external block building via builder API, which would bypass the mempool.

signed_read check is now done as part of 2718 decoding, which happens in:

• p2p tx gossip (before pool admission).
• eth_sendRawTransaction (before pool admission).
• Block body RLP decode — both for locally-executed blocks and peer-received blocks, via Engine API or p2p.
• Any future codepath that decodes RLP tx bytes.

eth_call uses a special purpose recover_raw_seismic_call_tx function which allows signed_read=true txs.

Side note

In send_raw_transaction, the TypedData arm now decodes the EIP-712 payload into a SeismicTxEnvelope, re-encodes as RLP, and delegates to EthTransactions::send_raw_transaction(bytes). This makes sure all ingestion paths go through the 2718 decoding function. Also added a TODO mentioning that this ingestion path is not needed, and we could update our clients to send via the Bytes path directly.

More generally, this is a first step in the right direction, but I think the even cleaner design is to enforce signed_reads cryptoraphically instead. See the "Future hard-fork requiring change to SeismicTx" section in SeismicSystems/seismic-alloy#104

[github-actions]

Based on my analysis of the diff and the commit message, I can now provide a comprehensive review.

Moves signed-read validation from mempool to decoder level to close replay attack vulnerability.

The changes look correct and address an important security issue where attackers could replay intercepted eth_call signed-read payloads as state-changing transactions by bypassing mempool validation through alternative ingress paths.

Phase 1
No critical issues found. The security fix properly:

• Unifies all signed-tx ingress through Decodable2718 pipeline with decoder-level validation
• Uses permissive decode_2718_permit_seismic_calls only for legitimate eth_call paths
• Removes the bypassed mempool validation that was the security gap

Phase 2

• Cargo.toml:787 — The TODO comment mentions this temporarily points to seismic-alloy PR Remove error in Reth logs #104. The dependency update should be finalized to the merged commit before landing this PR.
• crates/seismic/txpool/src/validator.rs:107 — The TODO about moving recent_block_hash and expires_at_block validation to consensus level is important. These checks can currently be bypassed via builder API or other non-mempool paths, similar to the signed-read issue that was just fixed.

The refactoring successfully consolidates transaction handling by removing the parallel SeismicTransaction trait and send_typed_data_transaction method, while the security improvement ensures decode-time invariants apply uniformly across all transaction ingress paths.