Step 1 of 2 for hosting a large HTML file that is already on disk, without sending its bytes through the model's context. Returns a short-lived signed upload URL; upload the file to it with a direct HTTP PUT (Content-Type: text/html), then call finalize_upload with the returned upload_path and a doc name. Requires a shell and unrestricted outbound HTTP, so the PUT will not succeed where outbound network access is restricted; host_html_content accepts the HTML inline and works in any environment.