🧪 Add test for EnergyPattern.gen_string

[akutuva21]

🎯 What: The gen_string method of the EnergyPattern class in bionetgen/modelapi/structs.py was previously untested. This PR addresses that testing gap.
📊 Coverage: The scenario where gen_string() successfully formats the internal pattern and expression into a BNGL compliant string is now covered.
✨ Result: Test coverage is improved, adding a safety net for any future refactoring of EnergyPattern and ensuring the correctness of generated strings for energy patterns.

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PR created automatically by Jules for task 17860189420800755178 started by @akutuva21

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