fix(compat): return error on timeout instead of nil

exec.CommandContext kills the process when the deadline expires, and
that kill surfaces as an *exec.ExitError on the signal path. The
deadline check was after the ExitError check, so timeouts silently
returned (res, nil) with a -1 exit code — masking hangs as clean
non-zero exits. The package doc already promised timeouts return
errors; this just lines the code up with the contract.

Surfaced by the new TestRun_TimeoutReturnsError in compat_test.go.

Co-Authored-By: Paperclip <[email protected]>

Author: LeadGoEngineer

compat/compat.go

@@ -114,14 +114,20 @@ func (r Runner) Run(ctx context.Context, args ...string) (Result, error) {
 		res.ExitCode = 0
 		return res, nil
 	}
+	// Timeout check first. exec.CommandContext kills the process when
+	// the deadline expires, which surfaces as an *exec.ExitError on the
+	// signal path. The package contract promises a non-nil error on
+	// timeout, so we must detect that case before falling through to
+	// the ExitError handler — otherwise a hung CLI looks like a clean
+	// non-zero exit to the caller.
+	if errors.Is(runCtx.Err(), context.DeadlineExceeded) {
+		return res, fmt.Errorf("compat: %s timed out after %s", r.Binary, timeout)
+	}
 	var exitErr *exec.ExitError
 	if errors.As(err, &exitErr) {
 		res.ExitCode = exitErr.ExitCode()
 		return res, nil
 	}
-	if errors.Is(runCtx.Err(), context.DeadlineExceeded) {
-		return res, fmt.Errorf("compat: %s timed out after %s", r.Binary, timeout)
-	}
 	return res, fmt.Errorf("compat: failed to run %s: %w", r.Binary, err)
 }