fix: initialize event ID when creating compaction events

PiperOrigin-RevId: 934305101

Author: google-genai-bot

core/src/main/java/com/google/adk/summarizer/LlmEventSummarizer.java

@@ -100,6 +100,7 @@ public Maybe<Event> summarizeEvents(List<Event> events) {
                         .map(
                             compaction ->
                                 Event.builder()
+                                    .id(Event.generateEventId())
                                     .author("user")
                                     .actions(EventActions.builder().compaction(compaction).build())
                                     .invocationId(Event.generateEventId())

core/src/test/java/com/google/adk/summarizer/LlmEventSummarizerTest.java

@@ -100,6 +100,8 @@ public void summarizeEvents_success() {
                 .get(0)
                 .text())
         .hasValue("Summary");
+    assertThat(compactedEvent.id()).isNotNull();
+    assertThat(compactedEvent.id()).isNotEmpty();
     assertThat(compactedEvent.author()).isEqualTo("user");
     assertThat(compactedEvent.actions()).isNotNull();
     assertThat(compactedEvent.actions().compaction()).isPresent();