Add test case for validate JSON diags

internal/command/validate_test.go

@@ -577,6 +577,22 @@ func TestValidate_json(t *testing.T) {
 	}
 }
 
+func TestValidate_ensure_json_diags(t *testing.T) {
+	output, code := setupTest(t, "validate-invalid", "-json", "-var", "foo")
+
+	if code != 1 {
+		t.Fatalf("Should have failed: %d\n\n%s", code, output.Stderr())
+	}
+
+	gotString := output.Stdout()
+
+	var got map[string]any
+	err := json.Unmarshal([]byte(gotString), &got)
+	if err != nil {
+		t.Fatalf("Test did not produce valid JSON: %s", err)
+	}
+}
+
 func TestValidateWithInvalidListResource(t *testing.T) {
 	td := t.TempDir()
 	cases := []struct {