Function update_context_windows 

pub async fn update_context_windows(
    config: &mut Config,
    provider_override: Option<&str>,
    dry_run: bool,
    fetch_fn: Option<Box<dyn for<'a> Fn(&'a str, &'a ModelProviderConfig) -> Pin<Box<dyn Future<Output = Option<usize>> + Send + 'a>> + Send + Sync>>,
) -> Result<usize>

Update context_window in config.toml from provider /models endpoints. Returns the number of providers updated.