FEAT: add fields calc expressions and update fields distribution extension

[gmalinve]

Description

From 2026.1 there is a new API method to get all fields calc. expressions and I also took the opportunity to refacto the fields calc class + add new tests that didn't exist.

Issue linked

#6995

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).

[gmalinve]

@SMoraisAnsys @Samuelopez-ansys is there a reason why we don't have anymore fields_calculator tests? I remember we had them in the past. Am I missing something?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.07%. Comparing base (a123a1c) to head (f0ca145).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6996      +/-   ##
==========================================
- Coverage   83.51%   82.07%   -1.45%     
==========================================
  Files         254      254              
  Lines       77338    77348      +10     
==========================================
- Hits        64590    63482    -1108     
- Misses      12748    13866    +1118     

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[SMoraisAnsys]

LGTM, I left some minor comment mainly over:

1. Splitting a test into 3 parts for readability: setup, exercise, assert (we don't need a teardown)
2. Asking to move the patchs outside of the test content (as decorator) to ease readability

Thanks again for this changes @gmalinve

[Samuelopez-ansys]

LGTM