fix(plugin): ensure mocker.stub is not detected as a coroutine function

[CyberneticX-Tech]

Resolves #375

Because mocker.stub() constructs a MagicMock(spec=lambda *args, **kwargs: None), isinstance(stub, types.FunctionType) evaluates to True. When inspect.iscoroutinefunction(stub) is called, it inspects stub.__code__.co_flags & CO_COROUTINE. Accessing stub.__code__ creates a child MagicMock, whose bitwise & operation also returns a MagicMock, which evaluates to truthy, incorrectly marking the synchronous stub as a coroutine function.

Assigning fn.__code__ to stub.__code__ ensures stub retains the real code flags of a standard synchronous function while preserving all other MagicMock behaviors.