Reject Invalid Nonce Transactions

Implement logic in the mempool to reject transactions with nonces lower than the sender's current committed nonce and evict any such transactions already in the queue. If a client submits a transaction with a nonce that has already been used, it indicates either a replay attack, a client error, or that the client's view of the nonce is stale. Immediately reject such transactions with an appropriate error code indicating invalid nonce. Periodically scan the mempool for queued transactions whose nonces have been superseded by committed transactions and evict them from the queue, freeing resources. This prevents the mempool from accumulating stale transactions that can never succeed and ensures clients receive timely feedback about nonce issues so they can correct their transaction submission logic.

Quarter: Q2

No files or tests are identified in the issue. Start by locating the mempool transaction-admission and queue-management entry points, then inspect existing nonce and error handling; done means lower-than-committed-nonce submissions are rejected with the invalid-nonce code and superseded queued transactions are periodically evicted.