Add __matmul__ support to MatrixExpr

Implements the __matmul__ method for MatrixExpr, enabling matrix multiplication using the @ operator and ensuring the result is returned as a MatrixExpr instance.

Author: Zeroto521

src/pyscipopt/matrix.pxi

@@ -98,7 +98,10 @@ class MatrixExpr(np.ndarray):
 
     def __rsub__(self, other):
         return super().__rsub__(other).view(MatrixExpr)
-    
+
+    def __matmul__(self, other):
+        return super().__matmul__(other).view(MatrixExpr)
+
 class MatrixGenExpr(MatrixExpr):
     pass