Add XPRESS support to the MathOpt Python API

[jaxor24]

What

Exposes the existing Xpress MathOpt backend through the MathOpt Python API.

The proto and C++ layers already support Xpress
(SOLVER_TYPE_XPRESS, XpressParametersProto, XpressParameters in
cpp/parameters.{h,cc}, and the SolveParametersProto.xpress field), and the
native backend ships in the wheel. Only the Python layer was missing the
corresponding surface, so mathopt.solve(model, SolverType.XPRESS) was not
usable from Python.

Changes

• parameters.py
  • Add SolverType.XPRESS (maps to SOLVER_TYPE_XPRESS).
  • Add the XpressParameters wrapper (name/value controls), mirroring
    GurobiParameters and the C++ XpressParameters struct.
  • Add the SolveParameters.xpress field and serialize it in to_proto().
• init_arguments.py
  • Add the StreamableXpressInitArguments placeholder and the corresponding
    StreamableSolverInitArguments.xpress field, for parity with the other
    solvers.
• mathopt.py
  • Re-export XpressParameters and StreamableXpressInitArguments (required by
    the mathopt_test.py public-API check).
• parameters_test.py
  • Add an XpressParameters.to_proto test and an xpress case in
    test_to_proto_with_specifics; test_solver_type_round_trip now covers the
    new enum member automatically.
• BUILD.bazel
  • Add //ortools/math_opt/solvers:xpress_py_pb2 to the parameters library
    and parameters_test (the py_proto target already existed).

Testing

Validated against the published OR-Tools 9.15 wheel with Xpress 9.9:

• parameters_test.py — 17/17 pass (includes the new Xpress coverage).
• init_arguments_test.py — 5/5 pass.
• End-to-end: a binary MIP solved through the public API with
  SolverType.XPRESS and XpressParameters(param_values={"PRESOLVE": "0"})
  returned TerminationReason.OPTIMAL with the expected solution; the native
  Xpress backend loaded and applied the solver-specific control.

No changes to the C++/proto layers.

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[jaxor24]

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