BREAKING_CHANGE: [vertexai] Change the unmodifiable list to immutable…

… list for getHistory. (#10606)

PiperOrigin-RevId: 618183004

Co-authored-by: Zhenyi Qi <zhenyiqi@google.com>

java-vertexai/google-cloud-vertexai/src/main/java/com/google/cloud/vertexai/generativeai/ChatSession.java

@@ -27,9 +27,9 @@
 import com.google.cloud.vertexai.api.GenerationConfig;
 import com.google.cloud.vertexai.api.SafetySetting;
 import com.google.cloud.vertexai.api.Tool;
+import com.google.common.collect.ImmutableList;
 import java.io.IOException;
 import java.util.ArrayList;
-import java.util.Collections;
 import java.util.List;
 import java.util.Optional;
 
@@ -210,9 +210,9 @@ private void checkFinishReasonAndRemoveLastContent(GenerateContentResponse respo
   /**
    * Returns the history of the conversation.
    *
-   * @return an unmodifiable history of the conversation.
+   * @return a history of the conversation as an immutable list of {@link Content}.
    */
-  public List<Content> getHistory() {
+  public ImmutableList<Content> getHistory() {
     try {
       checkLastResponseAndEditHistory();
     } catch (IllegalStateException e) {
@@ -225,7 +225,7 @@ public List<Content> getHistory() {
       }
       throw e;
     }
-    return Collections.unmodifiableList(history);
+    return ImmutableList.copyOf(history);
   }
 
   /**