fix: Make sure each partial event has different timestamp

PiperOrigin-RevId: 778148175

Author: DeanChensj

src/google/adk/flows/llm_flows/base_llm_flow.py

@@ -16,6 +16,7 @@
 
 from abc import ABC
 import asyncio
+import datetime
 import inspect
 import logging
 from typing import AsyncGenerator
@@ -320,6 +321,7 @@ async def _run_one_step_async(
       ):
         # Update the mutable event id to avoid conflict
         model_response_event.id = Event.new_id()
+        model_response_event.timestamp = datetime.datetime.now().timestamp()
         yield event
 
   async def _preprocess_async(