Check images before a LinkedIn ad is built

Checks image URLs against LinkedIn's sponsored-content specs before anything is created: reachable over https, an accepted file type, under the size limit, and either 1.91:1 (1200×627) or square (1200×1200) so the feed does not choose its own crop. Use when: the user supplies creative URLs, and always before linkedin_create_image_campaign or linkedin_add_creative. An image that fails here fails mid-way through a create, after the campaign has already been made — which leaves half a campaign behind. Do not treat a warning as a blocker: a non-standard ratio uploads fine and simply gets cropped, and the user may be happy with that. Errors are blockers; warnings are the user's call. Do not use it for video — LinkedIn's video specs are not checked here. Returns per-URL content type, byte size, dimensions, which recommended size it matches, and separate error and warning lists. Costs nothing and downloads only the file header. If dimensions cannot be read, say so plainly rather than assuming the image is fine.