Creates a new message with attached files and streams the LLM response as NDJSON

The multipart form of the message endpoint, for a message the user attached files to. Each file is read on arrival into text and tables and stored with the message, so the turn — and every later turn in the conversation — can reason about it; the original bytes are not retained. Accepts .xlsx, .xlsm, .csv, .docx, .docm and .pdf up to 15 MB each. A file that cannot be read does not fail the message: the reason is recorded against it and Watts is told, because an unreadable upload is something to tell the user about rather than an error to throw. The response body is newline-delimited JSON, exactly as the JSON form of this endpoint returns. Requires the VirtualEnergyManager feature flag.