CI: Temporary fix for vtk-osmesa

[SMoraisAnsys]

Description

Pin vtk-osmesa version. Seems like something went wrong on vtk side as CI was using vtk-osmesa-900.548.751, see https://github.com/ansys/pyaedt/actions/runs/16275229761/job/45961814797

Issue linked

Related to #5524

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).

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.41%. Comparing base (5a82bc5) to head (d9d5aa8).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6407       +/-   ##
===========================================
+ Coverage   38.76%   85.41%   +46.64%     
===========================================
  Files         175      175               
  Lines       66282    66282               
===========================================
+ Hits        25696    56615    +30919     
+ Misses      40586     9667    -30919     

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