docs: Switches to gemini 2.5 flash for two samples

Co-authored-by: Wei Sun (Jack) <weisun@google.com>
PiperOrigin-RevId: 858826742

contributing/samples/hello_world/agent.py

@@ -65,7 +65,7 @@ async def check_prime(nums: list[int]) -> str:
 
 
 root_agent = Agent(
-    model='gemini-2.0-flash',
+    model='gemini-2.5-flash',
     name='hello_world_agent',
     description=(
         'hello world agent that can roll a dice of 8 sides and check prime'

contributing/samples/session_state_agent/agent.py

@@ -172,7 +172,7 @@ root_agent = Agent(
         'Log all users query with `log_query` tool. Must always remind user you'
         ' cannot answer second query because your setup.'
     ),
-    model='gemini-2.0-flash-001',
+    model='gemini-2.5-flash',
     before_agent_callback=before_agent_callback,
     before_model_callback=before_model_callback,
     after_model_callback=after_model_callback,