Function fetch_documentation_text 

pub async fn fetch_documentation_text(
    client: &Client,
    key: &str,
    library_id: &str,
    query: Option<&str>,
) -> Result<String, Context7Error>

Fetches documentation for library_id as raw plain text (markdown).

Uses type=txt. Returns the raw response body as a String. Returns Err(Context7Error) on HTTP errors to enable retry in run_with_retry.