[BUG] Variable.VariableType is not normalized to VType (str/bytes/enum coexist)

[jackthepunished]

Describe the bug
Variable.VariableType is not normalized, so it can hold three different representations of the same type:

• a VType member when built through the Python API with VType.INTEGER / INTEGER;
• a plain str ("I") when the problem comes from Problem.readMPS / Problem.read — the parser builds var_types with dtype='str' and _from_data_model passes each element straight to addVariable(vtype=...);
• bytes (b"I") when a caller passes them — addVariable and setVariableType accept any value unchecked.

Downstream code copes ad hoc: Problem.IsMIP tests in ("I", "S", b"I", b"S") and solver.is_mip branches on isinstance(var_types[0], bytes). VType("I") works because VType is a str enum, but VType(b"I") raises ValueError.

Steps/Code to reproduce bug

from cuopt.linear_programming.problem import Problem, VType, INTEGER

p = Problem()
a = p.addVariable(vtype=INTEGER)
b = p.addVariable(vtype="I")
c = p.addVariable(vtype=b"I")
print([type(v.VariableType) for v in (a, b, c)])
# [<enum 'VType'>, <class 'str'>, <class 'bytes'>]

q = Problem.readMPS("<any MIP>.mps")
print(type(q.getVariables()[0].VariableType))   # <class 'str'>

Expected behavior
VariableType is always a VType member, normalized in Variable.__init__ / setVariableType (decoding bytes, then VType(value)), so IsMIP, solver.is_mip, and display code do not each need to handle the variants.

Environment details

• From source on main (Problem.addVariable, Problem._from_data_model, io/parser_wrapper.pyx); not environment-specific.

Additional context
Surfaced while adding __repr__ in #1400, which decodes bytes before VType(...) until this is normalized.