Add additional subprocess diagnostics for the audita interface

internal/adapters/subprocess/run_test.go

@@ -122,6 +122,35 @@ func TestRunFailureRedactsSensitiveTail(t *testing.T) {
 	}
 }
 
+func TestRunFailureAddsBadDescriptorHint(t *testing.T) {
+	exe, err := os.Executable()
+	if err != nil {
+		t.Fatalf("os.Executable() error = %v", err)
+	}
+
+	dir := t.TempDir()
+	req := RunRequest{
+		Executable: exe,
+		Args:       []string{"-test.run=TestSubprocessHelper", "--", "failbadfd"},
+		EnvOverrides: map[string]string{
+			"GO_WANT_SUBPROCESS_HELPER": "1",
+		},
+		StdoutLogPath: filepath.Join(dir, "stdout.log"),
+		StderrLogPath: filepath.Join(dir, "stderr.log"),
+	}
+
+	_, err = Run(context.Background(), req)
+	if err == nil {
+		t.Fatal("Run() error = nil, want non-nil")
+	}
+	if !strings.Contains(err.Error(), "Bad file descriptor") {
+		t.Fatalf("error = %q, want stderr tail content", err.Error())
+	}
+	if !strings.Contains(err.Error(), "stderr stream appears invalid in child process") {
+		t.Fatalf("error = %q, want bad-descriptor hint", err.Error())
+	}
+}
+
 func TestRunTimeout(t *testing.T) {
 	exe, err := os.Executable()
 	if err != nil {
@@ -329,6 +358,9 @@ func TestSubprocessHelper(t *testing.T) {
 		key := os.Getenv("SUBPROCESS_HELPER_ENV_KEY")
 		_, _ = os.Stderr.WriteString("secret:" + os.Getenv(key) + "\n")
 		os.Exit(4)
+	case "failbadfd":
+		_, _ = os.Stderr.WriteString("OSError: [Errno 9] Bad file descriptor\n")
+		os.Exit(120)
 	case "sleep":
 		time.Sleep(500 * time.Millisecond)
 		os.Exit(0)