Xpress: empty bound check not working

[Mizux]

For SCIP and GLPK the following test pass since integer round leading to empty bound is correctly managed

We should try to improve xpress_solver to have the same behaviour...

or-tools/ortools/math_opt/solver_tests/mip_tests.cc

Lines 141 to 157 in 4336f9f

	TEST_P(SimpleMipTest, FractionalBoundsContainNoInteger) {
	if (GetParam().solver_type == SolverType::kGurobi) {
	// TODO(b/272298816): Gurobi bindings are broken here.
	GTEST_SKIP() << "TODO(b/272298816): Gurobi bindings are broken here.";
	}
	if (GetParam().solver_type == SolverType::kXpress) {
	// Xpress rounds bounds of integer variables on input, so the bounds
	// specified here result in [1,0]. Xpress also checks that bounds are
	// not contradicting, so it rejects creation of such a variable.
	GTEST_SKIP() << "Xpress does not support contradictory bounds.";
	}
	Model model;
	const Variable x = model.AddIntegerVariable(0.5, 0.6, "x");
	model.Maximize(x);
	EXPECT_THAT(Solve(model, GetParam().solver_type),
	IsOkAndHolds(TerminatesWith(TerminationReason::kInfeasible)));
	}

ref: GLPK implem...

or-tools/ortools/math_opt/solvers/glpk_solver.cc

Lines 1782 to 1809 in 4336f9f

	std::optional<SolveResultProto> GlpkSolver::EmptyIntegerBoundsResult() {
	const int num_cols = glp_get_num_cols(problem_);
	for (int c = 1; c <= num_cols; ++c) {
	if (!variables_.IsInteger(problem_, c)) {
	continue;
	}
	const double lb = variables_.unrounded_lower_bounds[c - 1];
	const double ub = variables_.unrounded_upper_bounds[c - 1];
	if (lb > ub) {
	// Unrounded bounds are inverted; this case is covered by
	// ListInvertedBounds(). We don't want to depend on the order of calls of
	// the two functions here so we exclude this case.
	continue;
	}
	if (std::ceil(lb) <= std::floor(ub)) {
	continue;
	}
	// We found a variable with empty integer bounds (that is lb <= ub but
	// ceil(lb) > floor(ub)).
	return ResultForIntegerInfeasible(
	/*is_maximize=*/glp_get_obj_dir(problem_) == GLP_MAX,
	/*bad_variable_id=*/variables_.ids[c - 1],
	/*lb=*/lb, /*ub=*/ub);
	}
	return std::nullopt;
	}

or-tools/ortools/math_opt/solvers/glpk_solver.cc

Lines 1090 to 1098 in 4336f9f

	// Deal with empty integer bounds that result in inverted bounds due to bounds
	// rounding.
	{ // Limit scope of `result`.
	std::optional<SolveResultProto> result = EmptyIntegerBoundsResult();
	if (result.has_value()) {
	RETURN_IF_ERROR(set_solve_time(result.value()));
	return std::move(result).value();
	}
	}