[REQ][Python] Remove Optional[] from from_dict/from_json method in models

Is your feature request related to a problem? Please describe.

This generated from_dict method returns None only if None is passed as obj.

https://github.com/OpenAPITools/openapi-generator/blob/c96d3088c506f6af09b095e7b25828fce17985dc/samples/client/echo_api/python/openapi_client/models/pet.py#L101-L118

So I need to check its return value every time even if I actually passing non-None value...

pet = Pet.from_dict(pet_dict)
assert(pet is not None)

# or

if pet is None:
  raise Exception("should not be reached here")

Describe the solution you'd like

Remove Optional[] from the obj parameter and the return value.

 def from_dict(cls, obj: Dict[str, Any]) -> Self: 
     """Create an instance of Pet from a dict""" 
     if not isinstance(obj, dict): 
         return cls.model_validate(obj) 

    ...

Describe alternatives you've considered

@overload also provides better typing:

    @overload
    def from_dict(cls, obj: None) -> None:
        ...

    @overload
    def from_dict(cls, obj: Dict[str, Any]) -> Self:
        ...

    @classmethod
    def from_dict(cls, obj: Optional[Dict[str, Any]]) -> Optional[Self]:
        """Create an instance of Pet from a dict"""
        if obj is None:
            return None

        ...

but I don't think this method should accept None value. I'm agree with checking None in caller side, like the commit which introduce None checking does.

Additional context

Start with samples/client/echo_api/python/openapi_client/models/pet.py, especially the from_dict and from_json entry points linked in the issue, and inspect how their annotations and None handling are generated. Locate the generator source responsible and check any existing Python model tests. Done means generated methods require non-None dictionaries and return the model type without Optional annotations, with callers handling None before invocation.