test(wecom): add dedupe helper coverage and align constant usage

Use wecomMaxProcessedMessages in tests and add a concurrent same-message test
to lock in race-safety behavior for markMessageProcessed.

pkg/channels/wecom_dedupe_test.go

@@ -9,15 +9,46 @@ func TestMarkMessageProcessed_DuplicateDetection(t *testing.T) {
 	var mu sync.RWMutex
 	processed := make(map[string]bool)
 
-	if ok := markMessageProcessed(&mu, &processed, "msg-1", 1000); !ok {
+	if ok := markMessageProcessed(&mu, &processed, "msg-1", wecomMaxProcessedMessages); !ok {
 		t.Fatalf("first message should be accepted")
 	}
 
-	if ok := markMessageProcessed(&mu, &processed, "msg-1", 1000); ok {
+	if ok := markMessageProcessed(&mu, &processed, "msg-1", wecomMaxProcessedMessages); ok {
 		t.Fatalf("duplicate message should be rejected")
 	}
 }
 
+func TestMarkMessageProcessed_ConcurrentSameMessage(t *testing.T) {
+	var mu sync.RWMutex
+	processed := make(map[string]bool)
+
+	const goroutines = 64
+	var wg sync.WaitGroup
+	wg.Add(goroutines)
+
+	results := make(chan bool, goroutines)
+	for i := 0; i < goroutines; i++ {
+		go func() {
+			defer wg.Done()
+			results <- markMessageProcessed(&mu, &processed, "msg-concurrent", wecomMaxProcessedMessages)
+		}()
+	}
+
+	wg.Wait()
+	close(results)
+
+	successes := 0
+	for ok := range results {
+		if ok {
+			successes++
+		}
+	}
+
+	if successes != 1 {
+		t.Fatalf("expected exactly 1 successful mark, got %d", successes)
+	}
+}
+
 func TestMarkMessageProcessed_RotationClearsMapAtBoundary(t *testing.T) {
 	var mu sync.RWMutex
 	processed := make(map[string]bool)