Prepare the conversation attachment folder and probe filename conflicts

Creates the caller's conversation subfolder (and the room share) if not yet present, then creates or returns a Draft folder for staging uploads. Simulates the rename-on-conflict logic for each requested filename without requiring the files to already exist. The returned folder is the Draft path — a staging area that is NOT shared with the room, so other participants cannot see in-progress uploads or files from aborted messages. Files are moved into the shared subfolder atomically when the attachment endpoint is called. Recommended client flow: 1. Call this endpoint to obtain the Draft folder path and predicted final names. 2. Display the predicted names as accessibility placeholders immediately. 3. Upload each file to a random temporary name (e.g. a UUID) inside the returned Draft folder — do NOT upload to the predicted final name. 4. Call the attachment endpoint with the temp path as filePath and the original desired name as fileName. The server moves the file from Draft into the shared subfolder, resolves any conflicts, and posts the message.