fix: Tracking headers not being added to default model use

PiperOrigin-RevId: 782931137

Author: Poggecci

core/src/main/java/com/google/adk/models/Gemini.java

@@ -193,8 +193,6 @@ public Builder vertexCredentials(VertexCredentials vertexCredentials) {
      *
      * @return A new {@link Gemini} instance.
      * @throws NullPointerException if modelName is null.
-     * @throws IllegalStateException if none of apiKey, VertexCredentials, or an explicit apiClient
-     *     is set.
      */
     public Gemini build() {
       Objects.requireNonNull(modelName, "modelName must be set.");
@@ -206,9 +204,11 @@ public Gemini build() {
       } else if (vertexCredentials != null) {
         return new Gemini(modelName, vertexCredentials);
       } else {
-        throw new IllegalStateException(
-            "Authentication strategy not set: Either apiKey, VertexCredentials, or an explicit"
-                + " apiClient must be provided.");
+        return new Gemini(
+            modelName,
+            Client.builder()
+                .httpOptions(HttpOptions.builder().headers(TRACKING_HEADERS).build())
+                .build());
       }
     }
   }

core/src/main/java/com/google/adk/models/LlmRegistry.java

@@ -32,22 +32,12 @@ public interface LlmFactory {
     BaseLlm create(String modelName);
   }
 
-  /** API client for interacting with the Gemini model. */
-  private static final Client geminiApiClient = Client.builder().build();
-
   /** Map of model name patterns regex to factories. */
   private static final Map<String, LlmFactory> llmFactories = new ConcurrentHashMap<>();
 
-  /** Returns the singleton instance of the Gemini API client. */
-  private static Client getGeminiApiClient() {
-    return geminiApiClient;
-  }
-
   /** Registers default LLM factories, e.g. for Gemini models. */
   static {
-    registerLlm(
-        "gemini-.*",
-        modelName -> Gemini.builder().modelName(modelName).apiClient(getGeminiApiClient()).build());
+    registerLlm("gemini-.*", modelName -> Gemini.builder().modelName(modelName).build());
   }
 
   /**