🧪 Add test coverage for ModelObj comment property

[akutuva21]

🎯 What: Added missing unit tests for the comment property (getter and setter) in bionetgen.modelapi.structs.ModelObj.

📊 Coverage: The new test_modelobj_comment test covers the following scenarios for the comment setter:

• Setting a string with a # (strips the hash)
• Setting a string with leading whitespace and a # (strips whitespace and hash)
• Setting a string without a #
• Setting a non-string value (e.g., an integer)

✨ Result: Improved test coverage and reliability for ModelObj property manipulation.

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PR created automatically by Jules for task 13863758712916026060 started by @akutuva21

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