Create a new message

Creates a new message in a chat room where you are a participant. The message will be sent from your user account. The mentions array is required and must contain at least one mention. Returns 404 if the chat room doesn't exist or you're not a participant (security-first: doesn't leak room existence). To mention users in the message: 1. Include the username with @ prefix in the content (e.g. "@Task Owner hello") 2. Add the corresponding user details in the mentions array 3. The mentions array is required and must contain at least one mention Note: The message is always sent as the authenticated user. You cannot specify sender_type or sender_id. If @ prefixes are missing from usernames in the content, they will be automatically added.