Updates metadata for multiple contracts

Patches metadata on up to 1,000 contracts in one atomic request. Each contract may receive different values. If a contract is missing or any key or value is invalid, nothing is written for any contract. Per contract this behaves exactly like patch-contract-metadata: keys omitted for that contract are left unchanged; null, empty, and whitespace-only values clear that key. A contract whose change a contract approval rule holds keeps its values and returns the approval_request_id its change waits on.