text: Title: [BUG] AttributeError error in merge_models function after document update
  Body: **Describe the bug**
  An error occurs in the merge_models function after defining a new field in the document update in the .update method that is not included in the pydantic model using the extra='allow' option.
  
  Follow the code example below and the exception
  
  **To Reproduce**
  ```python
  import asyncio
  
  from beanie import Document, init_beanie
  from pydantic import ConfigDict
  
  
  async def main():
      class DocumentTestModelWithModelConfigExtraAllow(Document):
          model_config = ConfigDict(extra='allow')
  
      await init_beanie(
          connection_string='mongodb://localhost:27017/beanie',
          document_models=[DocumentTestModelWithModelConfigExtraAllow],
      )
  
      doc = DocumentTestModelWithModelConfigExtraAllow()
      await doc.insert()
  
      await doc.update({"$set": {"my_extra_field": 12345}})
      assert doc.my_extra_field == 12345
  
  
  if __name__ == '__main__':
      asyncio.run(main())
  ```
  
  **Exception**
  ```
  Traceback (most recent call last):
    File "/home/myuser/Documents/.gits/myproject/test.py", line 402, in <module>
      asyncio.run(main())
    File "/home/myuser/.pyenv/versions/3.11.10/lib/python3.11/asyncio/runners.py", line 190, in run
      return runner.run(main)
             ^^^^^^^^^^^^^^^^
    File "/home/myuser/.pyenv/versions/3.11.10/lib/python3.11/asyncio/runners.py", line 118, in run
      return self._loop.run_until_complete(task)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "/home/myuser/.pyenv/versions/3.11.10/lib/python3.11/asyncio/base_events.py", line 654, in run_until_complete
      return future.result()
             ^^^^^^^^^^^^^^^
    File "/home/myuser/Documents/.gits/myproject/test.py", line 397, in main
      await doc.update({"$set": {"my_extra_field": 12345}})
    File "/home/myuser/.cache/pypoetry/virtualenvs/myproject-B1nOShX4-py3.11/lib/python3.11/site-packages/beanie/odm/actions.py", line 239, in wrapper
      result = await f(
               ^^^^^^^^
    File "/home/myuser/.cache/pypoetry/virtualenvs/myproject-B1nOShX4-py3.11/lib/python3.11/site-packages/beanie/odm/utils/state.py", line 85, in wrapper
      result = await f(self, *args, **kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "/home/myuser/.cache/pypoetry/virtualenvs/myproject-B1nOShX4-py3.11/lib/python3.11/site-packages/beanie/odm/documents.py", line 740, in update
      merge_models(self, result)
    File "/home/myuser/.cache/pypoetry/virtualenvs/myproject-B1nOShX4-py3.11/lib/python3.11/site-packages/beanie/odm/utils/parsing.py", line 27, in merge_models
      left_value = getattr(left, k)
                   ^^^^^^^^^^^^^^^^
    File "/home/myuser/.cache/pypoetry/virtualenvs/myproject-B1nOShX4-py3.11/lib/python3.11/site-packages/pydantic/main.py", line 856, in __getattr__
      raise AttributeError(f'{type(self).__name__!r} object has no attribute {item!r}')
  AttributeError: 'DocumentTestModelWithModelConfigExtraAllow' object has no attribute 'my_extra_field'
  ```
  