fix confirmed deployment risks

backend/internal/handler/payment_webhook_handler_test.go

@@ -97,3 +97,64 @@ func TestWebhookConstants(t *testing.T) {
 		assert.Equal(t, 200, webhookLogTruncateLen)
 	})
 }
+
+func TestWriteUnavailableResponse(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	tests := []struct {
+		name            string
+		providerKey     string
+		wantCode        int
+		wantContentType string
+		wantBody        string
+		checkJSON       bool
+		wantJSONCode    string
+		wantJSONMessage string
+	}{
+		{
+			name:            "wxpay returns JSON failure to trigger retry",
+			providerKey:     "wxpay",
+			wantCode:        http.StatusServiceUnavailable,
+			wantContentType: "application/json",
+			checkJSON:       true,
+			wantJSONCode:    "FAIL",
+			wantJSONMessage: "provider unavailable",
+		},
+		{
+			name:            "stripe returns 503 with empty body",
+			providerKey:     "stripe",
+			wantCode:        http.StatusServiceUnavailable,
+			wantContentType: "text/plain",
+			wantBody:        "",
+		},
+		{
+			name:            "easypay returns 503 plain text",
+			providerKey:     "easypay",
+			wantCode:        http.StatusServiceUnavailable,
+			wantContentType: "text/plain",
+			wantBody:        "provider unavailable",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			w := httptest.NewRecorder()
+			c, _ := gin.CreateTestContext(w)
+
+			writeUnavailableResponse(c, tt.providerKey)
+
+			assert.Equal(t, tt.wantCode, w.Code)
+			assert.Contains(t, w.Header().Get("Content-Type"), tt.wantContentType)
+
+			if tt.checkJSON {
+				var resp map[string]string
+				err := json.Unmarshal(w.Body.Bytes(), &resp)
+				require.NoError(t, err)
+				assert.Equal(t, tt.wantJSONCode, resp["code"])
+				assert.Equal(t, tt.wantJSONMessage, resp["message"])
+			} else {
+				assert.Equal(t, tt.wantBody, w.Body.String())
+			}
+		})
+	}
+}