BUG: `MatrixVariable.sum(axis=...)` return type is `MatrixVariable` not `MatrixExpr`

[Zeroto521]

Describe the bug
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MatrixVariable.sum(axis=...) should return MatrixExpr, but return MatrixVariable

To Reproduce
Steps to reproduce the behavior

from pyscipopt import Model

model = Model()
x = model.addMatrixVar((3, 2))
# MatrixVariable([Expr({Term(x1): 1.0, Term(x2): 1.0}),
#                 Expr({Term(x3): 1.0, Term(x4): 1.0}),
#                 Expr({Term(x5): 1.0, Term(x6): 1.0})], dtype=object)

type(x.sum(axis=1))
# pyscipopt.scip.MatrixVariable

x.sum(axis=1).vtype()  # MatrixVariable has vtype method
# File src/pyscipopt/scip.pxi:1887, in pyscipopt.scip.MatrixVariable.vtype()
# AttributeError: 'pyscipopt.scip.Expr' object has no attribute 'vtype'

Expected behavior
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MatrixVariable.sum(axis=...) should return MatrixExpr

Screenshots
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System

• OS: Windows 11
• Version: 24H2 (26100.4061)
• SCIP version: 9.2.4
• pyscipopt version: 5.7.1
• How did you install pyscipopt? pip via pypi

Additional context
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[Zeroto521]

It's similar to #1058