Send Assistant Message

Create-or-continue the conversation, queue this message's turn, and wake the thread's ConversationWorkflow (TEA-772: row first, then the contentless wake). Returns the thread and queued turn immediately; the answer streams over the thread's SSE stream, and the run id is the entity's to capture. A message sent while the previous turn still runs QUEUES — the inbox is the queue, durable and ordered; the retired busy 409 has no successor. A parked previous turn is voided at the entity's member open — its pending work superseded with the outcome recorded as unknown, one beat after the 201. If the orchestration service can't be reached the turn is honestly settled failed and the caller gets the 503 — never a message that silently goes nowhere.