[HOLD] Track variational solver convergence

[mickaelbegon]

Summary

• add a reusable solver-convergence assertion with an optional iteration ceiling
• report solver status and iteration count in assertion failures
• protect four variational-integrator variants against convergence and major iteration regressions

Rationale

Exact IPOPT iteration counts can vary across platforms and dependency versions. This first increment uses a deliberately broad ceiling of 100 iterations for problems that currently converge comfortably below it, detecting meaningful regressions without making CI unnecessarily brittle.

Validation

• MPLCONFIGDIR=/private/tmp/bioptim-mpl-990 python -m pytest tests/shard4/test_variational_integrator_examples.py -vv (4 passed)
• git diff --check

Progress toward #990; this does not close the repository-wide tracking issue.

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