Upload to signed URL

Receives a file at a one-time signed URL issued by the MCP request-media-upload-tool. No API key: the signature authorizes the request. Send the file as multipart/form-data in the media field, for example curl -F media=@launch.jpg "<upload_url>". The URL works once, until it expires (15 minutes by default); a second upload to it returns 409. The returned upload_token is then used as a media item, like a token from POST /uploads. Accepted files: JPEG, PNG, GIF and WebP images (HEIC and HEIF too when the server can convert them, stored as JPEG), MP4 and MOV videos, and PDF documents. Size caps: 10 MB per image (and at most about 67 megapixels, 8192 × 8192 pixels in total), 1 GB per video, 100 MB per PDF. Each network enforces its own, usually smaller, caps when the post is scheduled or published (GET /content-types). A file the server cannot read returns 422 with only message: Unable to read uploaded file.