[BUG] Python: Problem.addConstraint accepts the same Constraint twice #2326

In the Python API, Problem.addConstraint accepts the same Constraint object twice. The second call overwrites constr.index and appends the constraint to Problem.constrs again, so the problem has two rows for one Constraint, and both rows map to the second index.

from cuopt.linear_programming.problem import Problem

p = Problem("dup")
x = p.addVariable(lb=0, ub=1, name="x")
c = x <= 1
p.addConstraint(c)
p.addConstraint(c)  # accepted; c.index becomes 1 and p.constrs holds c twice

Expected: ValueError, with p.constrs and c.index left unchanged.

Same bug as the Java one in #2224 (fixed in #2226). The check belongs in Problem.addConstraint, because the native solver only receives a CSR matrix.

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