chore(vertexai): Add some tests

FUTURE_COPYBARA_INTEGRATE_REVIEW=#14330 from googleapis:librarian-20260402T153541Z 64dc127
PiperOrigin-RevId: 895405821

Author: BenjaminKazemi

vertexai/genai/client_test.go

@@ -739,3 +739,14 @@ func printResponse(resp *GenerateContentResponse) {
 	}
 	fmt.Println("---")
 }
+
+func TestNewGenAIClient(t *testing.T) {
+	ctx := context.Background()
+	client, err := NewGenAIClient(ctx, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if client == nil {
+		t.Error("client must not be nil")
+	}
+}