Check TikTok creative URLs before uploading

Checks a video or image URL before anything is uploaded: that it is reachable without a login, that it serves a format TikTok accepts, and that it is inside the size limit — then states the rules that can only be verified after upload. Use when: the user gives a URL for a creative, before tiktok_create_video_campaign or tiktok_upload_images. It costs nothing and it catches the failures that would otherwise appear as an opaque upload error. Do not treat a clean result as approval. Duration, aspect ratio and dimensions cannot be read from HTTP headers — TikTok reports them after upload, and the create tool checks them there. What this catches is the common half: an expired signed link, a file behind a login, a MOV where an MP4 was meant, a 700 MB export. Do not use it on a TikTok CDN link or a video already in the library; use tiktok_list_ad_videos for those, which gives the real dimensions. Returns per URL: reachable, content type, size, and what to fix. The rules are 5–60 seconds, 9:16 (or 1:1 / 16:9), under 500 MB, MP4 preferred.