🧪 Test improvement: Add test for Rule.side_string

[akutuva21]

🎯 What: Added a missing unit test for the previously untested side_string public method in the Rule class.
📊 Coverage: The new test covers empty lists, single-element lists, and multiple-element lists to verify the " + ".join() logic handles these scenarios correctly.
✨ Result: Increased test coverage for bionetgen/modelapi/structs.py and improved testing reliability.

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PR created automatically by Jules for task 5025066414790934300 started by @akutuva21

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