Time are messed up in Chainlit

[asvishnyakov]

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Describe the bug https://github.com/Chainlit/chainlit/blob/0e863aee98c030b85d67e0d9ed8ad6f532967820/backend/chainlit/data/chainlit_data_layer.py#L374-L377

Saves in UTC (with Z), but https://github.com/Chainlit/chainlit/blob/0e863aee98c030b85d67e0d9ed8ad6f532967820/backend/chainlit/data/chainlit_data_layer.py#L635

reads in local time (without Z)

and when system tries to save it again we get exception

2025-09-03 22:00:48 - Task exception was never retrieved
future: <Task finished name='Task-1150' coro=<ChainlitDataLayer.update_step() done, defined at /Users/dmitriy/Documents/GitHub/cookbook/resume-chat/.venv/lib/python3.13/site-packages/chainlit/data/utils.py:10> exception=ValueError("time data '2025-09-04T02:00:42.164000' does not match format '%Y-%m-%dT%H:%M:%S.%fZ'")>
Traceback (most recent call last):
  File "/opt/homebrew/Cellar/[email protected]/3.13.3/Frameworks/Python.framework/Versions/3.13/lib/python3.13/asyncio/tasks.py", line 304, in __step_run_and_handle_result
    result = coro.send(None)
  File "/Users/dmitriy/Documents/GitHub/cookbook/resume-chat/.venv/lib/python3.13/site-packages/chainlit/data/utils.py", line 25, in wrapper
    return await method(self, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dmitriy/Documents/GitHub/cookbook/resume-chat/.venv/lib/python3.13/site-packages/chainlit/data/chainlit_data_layer.py", line 388, in update_step
    await self.create_step(step_dict)
  File "/Users/dmitriy/Documents/GitHub/cookbook/resume-chat/.venv/lib/python3.13/site-packages/chainlit/data/utils.py", line 25, in wrapper
    return await method(self, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dmitriy/Documents/GitHub/cookbook/resume-chat/.venv/lib/python3.13/site-packages/chainlit/data/chainlit_data_layer.py", line 368, in create_step
    timestamp = datetime.strptime(created_at, ISO_FORMAT)
  File "/opt/homebrew/Cellar/[email protected]/3.13.3/Frameworks/Python.framework/Versions/3.13/lib/python3.13/_strptime.py", line 674, in _strptime_datetime
    tt, fraction, gmtoff_fraction = _strptime(data_string, format)
                                    ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/[email protected]/3.13.3/Frameworks/Python.framework/Versions/3.13/lib/python3.13/_strptime.py", line 453, in _strptime
    raise ValueError("time data %r does not match format %r" %
                     (data_string, format))
ValueError: time data '2025-09-04T02:00:42.164000' does not match format '%Y-%m-%dT%H:%M:%S.%fZ'

when returned back to data layer and it tries to update thread step

To Reproduce See #2486

Expected behavior UTC should be saved returned