The _standalone_send() function (used by the send_message tool for
proactive/scheduled sends) has the same bare `@<id> text` bug that
send() had before PR #44444. SimpleX's `@<x>` syntax resolves x as a
display name, not a contactId — the daemon silently drops messages
when it cannot find a contact named "6".
Use the structured `/_send @<id> json [...]` form, matching what
send_image, send_document, and the send() fix already use.
Fixes#46265
eb048772f6