BREAKING_CHANGE: [vertexai] Change the unmodifiable list to immutable list for getHistory. (#10606)

PiperOrigin-RevId: 618183004 Co-authored-by: Zhenyi Qi <zhenyiqi@google.com>

Author: copybara-service[bot]

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);
  }
 
  /**