fix: make sure export_glb is handling a single Polydata object

[RobPasMue]

Description

export_glb requires that the actor is a single Polydata. Multiblock's are not handled properly by PyVista's export_gltf function.

Issue linked

Closes #2030

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 unit tests.
• I have reviewed my changes before submitting this pull request.
• I have linked the issue or issues that are solved to the PR if any.
• I have assigned this PR to myself.
• I have added the minimum version decorator to any new backend method implemented.
• I have made sure that the title of my PR follows Conventional commits style (e.g. feat: extrude circle to cylinder)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.50%. Comparing base (b86542b) to head (cffe475).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2032      +/-   ##
==========================================
+ Coverage   90.45%   90.50%   +0.05%     
==========================================
  Files         136      136              
  Lines        9782     9787       +5     
==========================================
+ Hits         8848     8858      +10     
+ Misses        934      929       -5     

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