[mesheryctl] Add tests for model import command

[AdiAkhileshSingh15]

Summary of changes
This PR refactors certain functionalities into helper utils, adds tests for helper functions (to manage the utility function tests separately from the import command tests), and tests for import command. So far, I've added test scenarios for handling invalid files and empty files, as implemented by @lekaf974 in PR #12328. For additional testing, if someone could provide guidance on specific tests we need to include, I’d be happy to contribute further!

Notes for Reviewers

• This PR fixes [mesheryctl] Create Unit and Integration Tests for mesheryctl model import Command #12137

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[vishalvivekm]

@AdiAkhileshSingh15 Thank you for your contributions! Let's discuss this today, during Meshery Development call at 9:00 AM CT | 8:30 PM IST. Adding it as an agenda item to the meeting minutes || meeting link.