Function run_test_file 

pub async fn run_test_file(
    path: &Path,
    filter: Option<&str>,
    timeout_ms: u64,
    execution_cwd: Option<&Path>,
    cli_skill_dirs: &[PathBuf],
) -> Result<Vec<TestResult>, String>

Backwards-compatible single-file API used by harn dev.

Runs every test in one file on the current thread. The new scheduler uses per-test worker threads, but harn dev re-runs a single module in the foreground after each rebuild — the queueing machinery would add latency without parallelism to gain back, so we keep this path minimal.