Returns a short-lived **V4-signed GCS URL** for a single SOURCE file (PDF / XLSM / XLSX / DOC / ZIP) the carrier submitted for a SERFF filing. The link is intended for **display to the end user** — they click it in their browser to download the file.
**CRITICAL: DO NOT fetch this URL yourself.** Surface it to the user verbatim and stop. The URL is a signed link for the human's browser, not for the model. Fetching it pulls the entire source file (often tens of MB of PDF / XLSM) into your context window and serves no purpose the user did not already get from seeing the link.
Pair with `list_filing_source_files` to discover the file names first, then call this to mint a link. When you respond to the user, include the URL **and the `expires_at` timestamp** so they know how long they have to click — after that the link returns 403 and they'll need to ask for a fresh one.
Link properties: direct V4-signed GCS URL, expires after `ttl_seconds` (default 900 = 15 min, capped at 3600). Bypasses Cloud Run entirely. Intended for human clicks, NOT for the model to fetch.
Whitelist is dynamic, keyed off the actual contents of the filing's source-files directory — same set `list_filing_source_files` advertises. `file_name` must be a basename (no slashes, no `..`) AND must appear in the listing.
Returns `{ serff, file_name, url, expires_at, ttl_seconds, notice }`. The `notice` repeats the don't-fetch directive — include it in your response to the user too.
Connector