fix: Add missing __post_init__ run from function variant

Author: ilotoki0804

src/fieldenum/_fieldenum.py

@@ -431,6 +431,9 @@ def __init__(self, *args, **kwargs) -> None:
                     result = item._func(*bound.args, **bound.kwargs)
                     if result is not None:
                         raise TypeError("Initializer should return None.")
+
+                    post_init = getattr(self, "__post_init__", lambda: None)
+                    post_init()
         # fmt: on
 
         self._actual = ConstructedVariant