Edge case: duplicate taskId entries in one relayer_sendTransactionMultichain batch

## Summary

`process_send_transaction_multichain` validates each provided `taskId` against storage but does not detect **duplicate task IDs within the same request array** before processing (`src/methods/send_tx/multi.rs`).

## Behavior

The first item succeeds and persists the id; a later item with the same `taskId` fails with duplicate (`4214`) after partial relay, leaving the batch in a partially applied state.

## Suggested fix

Pre-scan the `items` slice for duplicate `taskId` values (when `Some`) and reject the entire request with `-32602` or `4214` before mutating state.

## References

- `src/methods/send_tx/multi.rs`