Assert the body a retry delivers, not only its status (closes #294)

TestProcessRetryTask_LargeBody_FetchFromDB and
TestProcessRetryTask_SuccessfulRetry checked only that the delivery
ended delivered, so a retry that sent an empty or wrong body still
passed. Both now compare the body received at the sink with the stored
event body, as the processNewTask large-body test already does.

Model: opus-5-5

internal/delivery/engine_integration_test.go

@@ -355,9 +355,14 @@ func TestProcessRetryTask_SuccessfulRetry(t *testing.T) {
 
 	s := newISetup(t)
 
+	var receivedBody string
+
 	ts := httptest.NewServer(
 		http.HandlerFunc(
-			func(w http.ResponseWriter, _ *http.Request) {
+			func(w http.ResponseWriter, r *http.Request) {
+				body, _ := io.ReadAll(r.Body)
+				receivedBody = string(body)
+
 				w.WriteHeader(http.StatusOK)
 			},
 		),
@@ -397,6 +402,8 @@ func TestProcessRetryTask_SuccessfulRetry(t *testing.T) {
 		context.TODO(), &task,
 	)
 
+	assert.Equal(t, event.Body, receivedBody)
+
 	iAssertStatus(t, s.WebhookDB, d.ID,
 		database.DeliveryStatusDelivered,
 	)
@@ -443,9 +450,14 @@ func TestProcessRetryTask_LargeBody_FetchFromDB(
 
 	s := newISetup(t)
 
+	var receivedBody string
+
 	ts := httptest.NewServer(
 		http.HandlerFunc(
-			func(w http.ResponseWriter, _ *http.Request) {
+			func(w http.ResponseWriter, r *http.Request) {
+				body, _ := io.ReadAll(r.Body)
+				receivedBody = string(body)
+
 				w.WriteHeader(http.StatusOK)
 			},
 		),
@@ -482,6 +494,8 @@ func TestProcessRetryTask_LargeBody_FetchFromDB(
 		context.TODO(), &task,
 	)
 
+	assert.Equal(t, largeBody, receivedBody)
+
 	iAssertStatus(t, s.WebhookDB, d.ID,
 		database.DeliveryStatusDelivered,
 	)