Increase utility test coverage

[mickaelbegon]

Summary

• add fast utility tests for Fourier approximation helpers
• cover solution pickle export duplicate-time handling
• cover pulse intensity sliding-window constraint padding and trimming

Note

• Recreated from Increase utility test coverage mickaelbegon/cocofest-pedalage#3 against pyomeca/cocofest:main.
• This branch includes the bioptim 3.4 migration commits because all recreated PRs target main.

Verification

• black . --exclude "external/*" --check -l120
• PYTHONPATH=/private/tmp/pyomeca-bioptim-fdafe4d:. MPLBACKEND=Agg OMP_NUM_THREADS=1 OPENBLAS_NUM_THREADS=1 MKL_NUM_THREADS=1 VECLIB_MAXIMUM_THREADS=1 /Users/mickaelbegon/miniconda3/envs/cocofest-bayes/bin/python -m pytest tests/shard1/test_utility_coverage.py -q
• PYTHONPATH=/private/tmp/pyomeca-bioptim-fdafe4d:. MPLBACKEND=Agg OMP_NUM_THREADS=1 OPENBLAS_NUM_THREADS=1 MKL_NUM_THREADS=1 VECLIB_MAXIMUM_THREADS=1 /Users/mickaelbegon/miniconda3/envs/cocofest-bayes/bin/python -m pytest --cov-report term-missing --cov=cocofest tests/shard1/test_models_dynamics_without_bioptim.py tests/shard1/test_ivp.py tests/shard1/test_fes_dynamics.py tests/shard1/test_utility_coverage.py -q

---

This change is 

[mickaelbegon]

I rebuilt this branch on top of the cleaned bioptim 3.4 migration PR so the diff is now limited to the added utility coverage test only.

Current scope:

• tests/shard1/test_utility_coverage.py

Validation on my side:

• python -m pytest tests/shard1/test_utility_coverage.py -q -> 5 passed