Add cloudpickle to core dependencies

[AlexanderFengler]

Summary

• cloudpickle is imported unconditionally at module level in ssms/dataset_generators/lan_mlp.py but was missing from pyproject.toml core dependencies
• This causes ImportError on fresh pip install ssm-simulators (without mlflow)
• CI never caught it because mlflow-skinny transitively installs cloudpickle via the dev extras

Test plan

• pip install ssm-simulators in a clean venv, then import ssms succeeds
• Existing tests still pass

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
unittests	92.29% <ø> (ø)

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