MiniZinc type error when using 'solve_with_API=True' in XOR linear CP model

[davidGG271]

When running the linear CP XOR model using the Python MiniZinc API (solve_with_API=True), the solver fails with a MiniZinc type error due to already defined variables. This issue does not occur when using the external solver execution (solve_with_API=False), even though both paths are expected to be handled uniformly via solve_for_ARX.

Steps to Reproduce:

1. Go to the following test file:
   tests/unit/cipher_modules/models/cp/mzn_models/mzn_xor_linear_model_test.py
2. In the method def test_find_one_xor_linear_trail_with_fixed_weight() modify the call to find_one_xor_linear_trail_with_fixed_weight as follows:
   trail = mzn.find_one_xor_linear_trail_with_fixed_weight(3, solve_external=False, solve_with_API=True)
3. Run the test
   pytest -v -s tests/unit/cipher_modules/models/cp/mzn_models/mzn_xor_linear_model_test.py::test_find_one_xor_linear_trail_with_fixed_weight

Expected Behavior:
The model should solve correctly using the MiniZinc Python API, in the same way it does when solve_with_API=False

Actual Behavior:
The test fails with the following MiniZinc error:
minizinc.error.MiniZincError: Error: type error: identifier rot_0_0_i already defined

[rony-ramos]

A draft PR has been opened with the proposed fix and a detailed explanation of the root cause and changes: #402