feat: add support for Gemma models in LlmRegistry

- Registered 'gemma-.*' models to use the Gemini builder.
- Added GemmaTest.java to verify valid and invalid model names.
- Restored main BUILD file and added GemmaTest target with env setup.

Fixes b/499032158

PiperOrigin-RevId: 893918083

Author: glaforge

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

@@ -16,6 +16,7 @@
 
 package com.google.adk.models;
 
+import com.google.common.annotations.VisibleForTesting;
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
 
@@ -38,6 +39,7 @@ public interface LlmFactory {
   static {
     registerLlm("gemini-.*", modelName -> Gemini.builder().modelName(modelName).build());
     registerLlm("apigee/.*", modelName -> ApigeeLlm.builder().modelName(modelName).build());
+    registerLlm("gemma-.*", modelName -> Gemini.builder().modelName(modelName).build());
   }
 
   /**
@@ -50,6 +52,17 @@ public static void registerLlm(String modelNamePattern, LlmFactory factory) {
     llmFactories.put(modelNamePattern, factory);
   }
 
+  /**
+   * Checks if the given model name matches any of the registered LLM factory patterns.
+   *
+   * @param modelName The model name to check.
+   * @return {@code true} if the model name matches at least one pattern, {@code false} otherwise.
+   */
+  @VisibleForTesting
+  static boolean matchesAnyPattern(String modelName) {
+    return llmFactories.keySet().stream().anyMatch(modelName::matches);
+  }
+
   /**
    * Returns an LLM instance for the given model name, using a cached or new factory-created
    * instance.

core/src/test/java/com/google/adk/models/GemmaTest.java

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.adk.models;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public class GemmaTest {
+
+  @Test
+  public void getLlm_withValidGemmaModels_succeeds() {
+    assertThat(LlmRegistry.matchesAnyPattern("gemma-4-26b-a4b-it")).isTrue();
+    assertThat(LlmRegistry.matchesAnyPattern("gemma-4-31b-it")).isTrue();
+  }
+
+  @Test
+  public void getLlm_withInvalidGemmaModels_throwsException() {
+    assertThat(LlmRegistry.matchesAnyPattern("not-a-gemma")).isFalse();
+    assertThat(LlmRegistry.matchesAnyPattern("gemma")).isFalse();
+  }
+}