FIX: Improve Variable management in Circuit

[Samuelopez-ansys]

Description

Variable class refactoring. It should improve the usage of Object oriented programming, mainly in Circuit designs.

Issue linked

Close #6710
Close ##6906

Checklist

• I have tested my changes locally.
• I have added necessary documentation or updated existing documentation.
• I have followed the coding style guidelines of this project.
• I have added appropriate tests (unit, integration, system).
• I have reviewed my changes before submitting this pull request.
• I have linked the issue or issues that are solved by the PR if any.
• I have agreed with the Contributor License Agreement (CLA).

[codecov]

Codecov Report

❌ Patch coverage is 87.27915% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.04%. Comparing base (27714c1) to head (ecbef2f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6742       +/-   ##
===========================================
+ Coverage   56.01%   81.04%   +25.02%     
===========================================
  Files         246      246               
  Lines       78112    78196       +84     
===========================================
+ Hits        43756    63373    +19617     
+ Misses      34356    14823    -19533     

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[SMoraisAnsys]

Very great job @Samuelopez-ansys I left some comments. Didn't check the tests changes yet, I'll do that later :)

[SMoraisAnsys]

We should probably log an error here (could also be the case in other changes where if not self._app is used)

[Samuelopez-ansys]

We allow the initialization of Variable class without AEDT, this is tested in test_variable.py

Not sure if we should log an error, because it means that the Variable object is updated, but the object can not update the variable in AEDT, because it is not connected... so instead of an error, it should be more a warning or info, what do you think?