Function fetch_one 

pub async fn fetch_one(
    url: &str,
    http: &HttpClient,
    params: &FetchParams,
    cache: &Cache,
) -> Result<(FeedResult, Vec<Warning>), RssError>

Fetch and parse a single feed, returning the FeedResult plus any non-fatal Warnings the parse surfaced (e.g. a content-extraction fallback). Callers aggregate the warnings into FetchOutput::warnings.