chore: Update Imagen tests to use Imagen 4

PiperOrigin-RevId: 832387149

Author: matthew29tang

src/test/java/com/google/genai/AsyncModelsTest.java

@@ -612,7 +612,7 @@ public void testGenerateImagesAsync(boolean vertexAI) throws Exception {
 
     // Act
     CompletableFuture<GenerateImagesResponse> responseFuture =
-        client.async.models.generateImages("imagen-3.0-generate-002", "Red skateboard", config);
+        client.async.models.generateImages("imagen-4.0-generate-001", "Red skateboard", config);
     GenerateImagesResponse response = responseFuture.join();
 
     // Assert
@@ -648,7 +648,7 @@ public void testUpscaleImageAsync(boolean vertexAI) throws Exception {
     // Act
     if (vertexAI) {
       CompletableFuture<UpscaleImageResponse> responseFuture =
-          client.async.models.upscaleImage("imagen-3.0-generate-002", image, "x2", config);
+          client.async.models.upscaleImage("imagen-4.0-upscale-preview", image, "x2", config);
       UpscaleImageResponse response = responseFuture.join();
 
       // Assert
@@ -661,7 +661,7 @@ public void testUpscaleImageAsync(boolean vertexAI) throws Exception {
                   client
                       .async
                       .models
-                      .upscaleImage("imagen-3.0-generate-001", image, "x2", config)
+                      .upscaleImage("imagen-4.0-upscale-preview", image, "x2", config)
                       .join());
       // Assert
       assertEquals(