userfront&backend test coverage 추가

backend/internal/response/error_response_test.go

@@ -58,6 +58,34 @@ func TestErrorWithDetailsResponseShape(t *testing.T) {
 	}
 }
 
+func TestErrorResponseShape(t *testing.T) {
+	app := fiber.New()
+	app.Get("/test", func(c *fiber.Ctx) error {
+		return Error(c, fiber.StatusUnauthorized, "invalid_session", "login required")
+	})
+
+	req := httptest.NewRequest(http.MethodGet, "/test", nil)
+	resp, err := app.Test(req)
+	if err != nil {
+		t.Fatalf("request failed: %v", err)
+	}
+
+	if resp.StatusCode != http.StatusUnauthorized {
+		t.Fatalf("unexpected status code: %d", resp.StatusCode)
+	}
+
+	body := parseBody(t, resp)
+	if body["error"] != "login required" {
+		t.Fatalf("unexpected error value: %v", body["error"])
+	}
+	if body["code"] != "invalid_session" {
+		t.Fatalf("unexpected code value: %v", body["code"])
+	}
+	if _, exists := body["details"]; exists {
+		t.Fatalf("details should be omitted when nil: %#v", body)
+	}
+}
+
 func TestStatusCodeMapping(t *testing.T) {
 	tests := []struct {
 		name     string