feat!: use @nullable fields in Event class

PiperOrigin-RevId: 878498806

Author: tilgalas

a2a/src/main/java/com/google/adk/a2a/RemoteA2AAgent.java

@@ -436,7 +436,7 @@ private boolean mergeAggregatedContentIntoEvent(Event event) {
       }
       Content aggregatedContent = Content.builder().role("model").parts(parts).build();
 
-      event.setContent(Optional.of(aggregatedContent));
+      event.setContent(aggregatedContent);
 
       ImmutableList.Builder<CustomMetadata> newMetadata = ImmutableList.builder();
       event.customMetadata().ifPresent(newMetadata::addAll);

a2a/src/main/java/com/google/adk/a2a/converters/ResponseConverter.java

@@ -76,7 +76,7 @@ private static Optional<Event> handleTaskUpdate(
       boolean isLastChunk = Objects.equals(artifactEvent.isLastChunk(), true);
 
       Event eventPart = artifactToEvent(artifactEvent.getArtifact(), context);
-      eventPart.setPartial(Optional.of(isAppend || !isLastChunk));
+      eventPart.setPartial(isAppend || !isLastChunk);
       // append=true, lastChunk=false: emit as partial, update aggregation
       // append=false, lastChunk=false: emit as partial, reset aggregation
       // append=true, lastChunk=true: emit as partial, update aggregation and emit as non-partial

a2a/src/test/java/com/google/adk/a2a/executor/AgentExecutorTest.java

@@ -361,7 +361,7 @@ private RequestContext createRequestContext() {
   public void process_statefulAggregation_tracksArtifactIdAndAppendForAuthor() {
     Event partial1 =
         Event.builder()
-            .partial(Optional.of(true))
+            .partial(true)
             .author("agent_author")
             .content(
                 Content.builder()
@@ -370,7 +370,7 @@ public void process_statefulAggregation_tracksArtifactIdAndAppendForAuthor() {
             .build();
     Event partial2 =
         Event.builder()
-            .partial(Optional.of(true))
+            .partial(true)
             .author("agent_author")
             .content(
                 Content.builder()
@@ -379,7 +379,7 @@ public void process_statefulAggregation_tracksArtifactIdAndAppendForAuthor() {
             .build();
     Event finalEvent =
         Event.builder()
-            .partial(Optional.of(false))
+            .partial(false)
             .author("agent_author")
             .content(
                 Content.builder()

core/src/main/java/com/google/adk/agents/BaseAgent.java

@@ -409,7 +409,7 @@ private Single<Optional<Event>> callCallback(
                                 .id(Event.generateEventId())
                                 .invocationId(invocationContext.invocationId())
                                 .author(name())
-                                .branch(invocationContext.branch())
+                                .branch(invocationContext.branch().orElse(null))
                                 .actions(callbackContext.eventActions())
                                 .content(content)
                                 .build());
@@ -426,7 +426,7 @@ private Single<Optional<Event>> callCallback(
                             .id(Event.generateEventId())
                             .invocationId(invocationContext.invocationId())
                             .author(name())
-                            .branch(invocationContext.branch())
+                            .branch(invocationContext.branch().orElse(null))
                             .actions(callbackContext.eventActions());
 
                     return Single.just(Optional.of(eventBuilder.build()));