refactor(backend): 清理 settle 残单预查死代码块——入参金额直接回填，fail-closed 语义不变

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/internal/service/pay_test.go

@@ -167,20 +167,15 @@ func TestHandleCallback_UnknownEventAcked(t *testing.T) {
 }
 
 // TestHandleCallback_ResidualAmountBackfill：残单兜底——建单时金额回填失败（amount_minor=0），
-// 入账前补查权威价成功 → 核对通过并回填。
+// webhook 自带权威金额（真实流恒非零）→ settle 事务内直接用入参回填并核对通过、入账续期。
+// settle 不再外呼 queryOrder，此处特意不配置 pay mock server（传 http://pay.invalid），
+// 证明回填全程无需外部查单。
 func TestHandleCallback_ResidualAmountBackfill(t *testing.T) {
 	db := testutil.SetupTestDB()
 	shop := testutil.CreateTestShop(db, "PAY015")
 	createPendingPurchase(t, db, shop.ID, "annual_standard", 0, "", "yanmei-otn-residual")
 
-	mux := http.NewServeMux()
-	mux.HandleFunc("GET /api/v2/orders/yanmei-otn-residual", func(w http.ResponseWriter, r *http.Request) {
-		fmt.Fprint(w, `{"data":{"order_no":"yanmei-otn-residual","status":"paid","amount_minor":299900,"currency":"CNY"}}`)
-	})
-	payServer := httptest.NewServer(mux)
-	defer payServer.Close()
-
-	svc := newTestPaySvc(db, payServer.URL)
+	svc := newTestPaySvc(db, "http://pay.invalid")
 	body := callbackBody("yanmei-otn-residual", "annual_standard", 299900)
 	ts, nonce, sign := signedCallbackArgs(body)
 	require.NoError(t, svc.HandleCallback(body, ts, nonce, sign))
@@ -188,21 +183,24 @@ func TestHandleCallback_ResidualAmountBackfill(t *testing.T) {
 	var p model.LicensePurchase
 	require.NoError(t, db.Where("out_trade_no = ?", "yanmei-otn-residual").First(&p).Error)
 	assert.Equal(t, "paid", p.Status)
-	assert.Equal(t, int64(299900), p.AmountMinor, "残单兜底应回填金额")
+	assert.Equal(t, int64(299900), p.AmountMinor, "残单兜底应用 webhook 权威金额回填")
 	assert.Equal(t, "CNY", p.Currency)
 	assert.NotNil(t, p.RenewedTo, "入账应落库续期后到期日")
 }
 
-// TestSettle_ResidualQueryFailFailsClosed：残单（amount_minor=0）且调用方自己也不知道金额
-// （amountMinor==0，如 reconcile 遇到 pay 侧字段异常）时，settle 内补查权威价若也失败
-// （pay 查单 500），必须 fail-closed：不入账、不续期、购买单维持 pending。
-func TestSettle_ResidualQueryFailFailsClosed(t *testing.T) {
+// TestSettle_ZeroAmountInputFailsClosed：settle 入参 amountMinor==0（真实流不应出现，
+// 防御性场景）时必须 fail-closed：guard 的 p.AmountMinor==0 分支兜底命中 ErrPayAmount，
+// 不入账、不续期、购买单维持 pending。settle 不再外呼 queryOrder 兜底补查，
+// 用调用计数断言零调用，防止该死代码路径回归。
+func TestSettle_ZeroAmountInputFailsClosed(t *testing.T) {
 	db := testutil.SetupTestDB()
 	shop := testutil.CreateTestShop(db, "PAY016")
 	createPendingPurchase(t, db, shop.ID, "annual_standard", 0, "", "yanmei-otn-residual-fail")
 
+	var queryCalls int
 	mux := http.NewServeMux()
 	mux.HandleFunc("GET /api/v2/orders/yanmei-otn-residual-fail", func(w http.ResponseWriter, r *http.Request) {
+		queryCalls++
 		w.WriteHeader(http.StatusInternalServerError)
 	})
 	payServer := httptest.NewServer(mux)
@@ -210,16 +208,17 @@ func TestSettle_ResidualQueryFailFailsClosed(t *testing.T) {
 
 	svc := newTestPaySvc(db, payServer.URL)
 	err := svc.settle("yanmei-otn-residual-fail", "annual_standard", 0, "", "alipay", time.Now())
-	assert.ErrorIs(t, err, ErrPayAmount, "补查也失败应 fail-closed")
+	assert.ErrorIs(t, err, ErrPayAmount, "入参金额为 0 应 fail-closed")
+	assert.Equal(t, 0, queryCalls, "settle 不应再外呼查单")
 
 	var p model.LicensePurchase
 	require.NoError(t, db.Where("out_trade_no = ?", "yanmei-otn-residual-fail").First(&p).Error)
-	assert.Equal(t, "pending", p.Status, "补查失败不得入账")
-	assert.Nil(t, p.RenewedTo, "补查失败不得续期")
+	assert.Equal(t, "pending", p.Status, "入参金额为 0 不得入账")
+	assert.Nil(t, p.RenewedTo, "入参金额为 0 不得续期")
 
 	var licCount int64
 	require.NoError(t, db.Model(&model.License{}).Where("shop_id = ?", shop.ID).Count(&licCount).Error)
-	assert.Equal(t, int64(0), licCount, "补查失败不得续期")
+	assert.Equal(t, int64(0), licCount, "入参金额为 0 不得续期")
 }
 
 // ---------- 回调：入账 ----------