Fix INDEPENDENT mechanism crash on duplicate cliques

[gghatano]

Fixes the ValueError: Cliques must be unique. raised when running
dpsynth.generate(..., discrete_config=IndependentConfig()) (see issue:
"IndependentConfig synthesis raises 'Cliques must be unique.'").

Cause

independent.run_mechanism builds measurements from initial_measurements
(the one-way marginals that data_generation_v2.generate always provides) and
then appends a freshly measured one-way marginal for every attribute, so the
list contains each one-way clique twice. When initial_potentials is not
None (e.g. the empty CliqueVector from constraints.get_initial_parameters
in the no-constraints case), potentials.expand([m.clique for m in measurements]) is called with duplicate cliques, which current mbi rejects.

Change

De-duplicate the clique list (order-preserving) before expand:

unique_cliques = list(dict.fromkeys(m.clique for m in measurements))
potentials = potentials.expand(unique_cliques)

measurements itself is unchanged, so the marginals fed to mirror_descent
and the privacy accounting are unaffected — only the clique set used to expand
the (zero) potentials is de-duplicated.

Verification

The repro in the issue (a 2-column categorical frame with
IndependentConfig()) now returns a synthetic DataFrame instead of raising.
MST/AIM are unaffected.

Fixes #2

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[ryan112358]

thanks for the bug fix, but can you remove this inline comment or reduce it to 1 line?

[ryan112358]

Can you also add test coverage for this case?

[gghatano]

Thanks for the review, @ryan112358! I've addressed both comments in 7d5ed01:

1. Inline comment — reduced it to a single line.
2. Test coverage — added test_duplicate_one_way_cliques_with_initial_potentials in independent_test.py. It exercises
   the exact failing path (an initial_potentials CliqueVector plus initial_measurements containing the one-way marginals,
   so the cliques measured in the loop duplicate them), which previously raised ValueError: Cliques must be unique.. I
   confirmed the new test fails without the fix and passes with it.

Both tests pass locally (2 passed).

Let me know if you'd like any further changes!