[HOLD] Test default multinode objective weight

[mickaelbegon]

Summary

• add a regression test ensuring multinode objectives default to a weight of 1
• document the behavior requested in penalty.weight=1 if not declared for multinode_objectives #692 in the test suite

Context

The underlying behavior was already corrected on master when objective and constraint weights were separated: MultinodeObjectiveList.add() now creates an ObjectiveWeight() whose default value is 1. This PR locks that behavior to prevent a future regression to the historical zero-weight behavior.

Validation

• MPLCONFIGDIR=/private/tmp/mplconfig-692 /Users/mickaelbegon/miniconda3/envs/bioptim/bin/python -m pytest tests/shard6/test_penalty.py::test_multinode_objective_default_weight_is_one -q
• 1 passed

Closes #692

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