[Version-unspecific] test: add simulation UTXO mempool actor

Summary

This MR adds a new simulation feature to probe mempool UTXO visibility behavior on UTXO chains, including ZEC. This MR adds a new simulation feature to probe mempool UTXO visibility behavior across UTXO chains, including ZEC.

Motivation

Signer safety depends on not reusing already-spent mempool inputs. Different chain RPCs (Zcashd/Zebra) can behave differently while transactions are still in mempool. This actor checks that behavior per chain and reports it.

Behavior

For each UTXO chain, the actor broadcasts a self-spend and checks whether spent inputs are still listed as spendable while the transaction is unconfirmed (getaddressutxos on ZEC, listunspent on other UTXO chains).

Result is reported as:

• pass if spent inputs are not returned
• fail (soft-fail-expected) if spent inputs are still returned
• inconclusive if tx confirms before mempool check

Summary by CodeRabbit

Release Notes

• New Features
  • Added UTXO mempool validation workflow to verify correct UTXO visibility during mempool transactions across blockchain chains.
  • Introduced chain-specific mempool validation with per-chain result tracking and aggregated reporting.
  • Added ZEC-specific UTXO retrieval support.
  • Added account lock-state checking capability for test management.