fix: Fix the incorrect timestamp in Event

PiperOrigin-RevId: 791274090

Author: google-genai-bot

core/src/main/java/com/google/adk/flows/llmflows/BaseLlmFlow.java

@@ -348,6 +348,9 @@ private Flowable<Event> runOneStep(InvocationContext context) {
                       .author(context.agent().name())
                       .branch(context.branch())
                       .build();
+              // Explicitly set the event timestamp to 0 so the postprocessing logic would generate
+              // events with fresh timestamp.
+              mutableEventTemplate.setTimestamp(0L);
 
               Flowable<Event> restOfFlow =
                   callLlm(context, llmRequestAfterPreprocess, mutableEventTemplate)