Rosetta: dedup base58-check memo decoding + nonce metadata construction shared by `User_command_info.to_transaction` and `Zkapp_command_info.T.to_transaction` into a single `nonce_and_memo_metadata` helper. As a side effect, the user-command path now always includes `nonce` in `/block` metadata (it was previously dropped whenever `memo = None` or decoding raised — same bug fixed for zkapps in #18810).
