feat: Add Generic[TModel] support for type-safe model attribute

[rodrigobnogueira]

This PR adds Generic support to StateMachine so type checkers can properly infer the model type.

Closes #515

What changed

• Added GenericStateMachineMetaclass that combines the existing metaclass with type(Generic)
• Made StateMachine inherit from Generic[TModel]
• Updated type hints for the model parameter and attribute
• Added tests for the new functionality

Why this is useful

Before this change, sm.model was typed as Any, which meant no autocomplete and no type checking for model attributes. Now you can do:

class MyModel:
    def __init__(self):
        self.state = None
        self.custom_field = "test"

class MyMachine(StateMachine[MyModel]):
    initial = State("Initial", initial=True)
    final = State("Final", final=True)
    go = initial.to(final)

sm = MyMachine(model=MyModel())
sm.model.custom_field  # type checker knows this exists

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud