Add validation of hinted methods on set and post initialise

[GDYendell]

As for Attributes:

• https://github.com/DiamondLightSource/FastCS/blob/main/src/fastcs/controllers/base_controller.py#L173
• https://github.com/DiamondLightSource/FastCS/blob/main/src/fastcs/controllers/base_controller.py#L248

We should add a helper method for checking for name clashes now because there will be a matrix of 3 things to check: attrbutes, methods and controllers each of which will have to check for clashes with itself and the other two.

def _check_for_name_clash(name: str):
    if name in self.__attributes:
        raise ...
    if name in self.__methods:
        raise ...
    if name in self.__sub_controllers:
        raise ...

and then call this and then do the specific checks for each type (e.g. attribute access mode)

Acceptance Criteria

• Hinted methods raise an exception if they are not introspected
• Assigning methods fails if an attribute, sub controller, or method of the same name already exists