CLPR: ABI for Middleware

Define the solidity ABI for
- Messages
- Contracts

Follow the pattern for other System Contracts. This may only appear in testing projects since the implementation is going to be a native system contract.

Start by examining the existing System Contract patterns in the repository and identify how their Solidity ABIs are represented, especially for Messages and Contracts. Define the corresponding middleware ABI in the testing projects if that is where native system-contract interfaces belong, and confirm that both requested interfaces follow the established pattern.

Written by the indexing model from the issue text.