fix(go): decouple mqtt message publishing from shutdown context

go/vehicle-gateway/internal/gateway/mqtt_client_test.go

@@ -69,6 +69,45 @@ func TestMQTTClientHandleBadPayloadPublishesOnlyRaw(t *testing.T) {
 	}
 }
 
+func TestMQTTClientUsesUncancelledMessageContextForReceivedMessage(t *testing.T) {
+	parent, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	resolver := &contextCheckingResolver{}
+	sink := &contextCheckingSink{}
+	client, err := NewMQTTClient(MQTTClientConfig{
+		EndpointName: "endpoint-a",
+		Broker:       "tcp://127.0.0.1:1883",
+		ClientID:     "test-client",
+		Topics:       []string{"/ytforward/shln/+"},
+		Sink:         sink,
+		Resolver:     resolver,
+		Logger:       slog.New(slog.NewTextHandler(testWriter{t: t}, nil)),
+	})
+	if err != nil {
+		t.Fatalf("NewMQTTClient() error = %v", err)
+	}
+
+	client.handleMessage(parent, "/ytforward/shln/dev1", []byte(`{
+	  "device":"LTEST000000000001",
+	  "time":"20260413100000",
+	  "data":{"METER_SPEED":52.3,"TOTAL_MILEAGE":123456.7}
+	}`))
+
+	if resolver.ctxErr != nil {
+		t.Fatalf("resolver saw cancelled context: %v", resolver.ctxErr)
+	}
+	if sink.rawCtxErr != nil {
+		t.Fatalf("raw publish saw cancelled context: %v", sink.rawCtxErr)
+	}
+	if sink.unifiedCtxErr != nil {
+		t.Fatalf("unified publish saw cancelled context: %v", sink.unifiedCtxErr)
+	}
+	if sink.rawCount != 1 || sink.unifiedCount != 1 {
+		t.Fatalf("raw=%d unified=%d", sink.rawCount, sink.unifiedCount)
+	}
+}
+
 func TestMQTTClientBuildOptionsLoadsTLSCertificates(t *testing.T) {
 	dir := t.TempDir()
 	caPath, certPath, keyPath := writeTestTLSMaterial(t, dir)