Function run_external_diagnostics 

pub async fn run_external_diagnostics(
    cfg: &ExternalToolsConfig,
    path: &Path,
    workspace_root: Option<&Path>,
) -> Vec<Diagnostic>

Run every enabled external tool against path and return their combined diagnostics. Tools run one after another rather than concurrently so a single caller gets one complete Vec back instead of having to merge two independently-arriving results.