SendMessage previously always sent from the first configured account
with no way to choose, so it silently used the wrong identity the
moment a second account existed. Add an optional account_id field to
SendMessageRequest; when set, resolve that specific account and fail
with a new AccountNotFound error if it does not exist, rather than
falling back to a different account. Omitting account_id preserves
the prior first-account behavior. Thread the selector through
ComposeRequest, resolve_sending_account, the CLI's mail send --account
flag, and regenerate the nuncio.v1 descriptor golden.
Refs #196, #170
James Maes