Function run_upstream_test_with_mode 

pub fn run_upstream_test_with_mode(
    rt: &RuntimeConfig,
    target: Option<&str>,
    mode: &str,
    timeout: Duration,
    json_output: bool,
) -> i32

Run upstream tests with an explicit mode (“proxy” or “tcp”).

Invalid modes fail closed with EXIT_CLI_PARSE_ERROR instead of silently falling back to either probe path. Normal callers pass a value from the typed UpstreamTestMode CLI enum (proxy/tcp), so this is defense-in-depth at the library boundary (including programmatic and Python-facing callers).