Add @disjoint_base decorator to UnaryExpr class

Applied the @disjoint_base decorator to the UnaryExpr class in scip.pyi to clarify its role in the type hierarchy.

Author: Zeroto521

src/pyscipopt/scip.pyi

@@ -2198,6 +2198,7 @@ class Term:
     def __lt__(self, other: object) -> bool: ...
     def __ne__(self, other: object) -> bool: ...
 
+@disjoint_base
 class UnaryExpr(GenExpr):
     def __init__(self, *args: Incomplete, **kwargs: Incomplete) -> None: ...