🧪 Add test coverage for untested Action.print_line() method

[akutuva21]

🎯 What: The print_line method of the Action class in bionetgen/modelapi/structs.py was completely untested, leading to potential unseen regressions. This gap has been addressed.

📊 Coverage: The new test_action_print_line explicitly verifies that print_line:

• Outputs the standard action string format correctly.
• Appropriately prepends a line label.
• Properly appends a comment when present.
• Correctly formats the string when both a label and a comment are configured.

✨ Result: A potential testing blindspot is closed, increasing overall robustness.

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PR created automatically by Jules for task 733674116129108694 started by @akutuva21

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