Improve polygon loader and update testlib.h

[pxsit]

• Added support for importing Polygon batch graders.
• Improved test file handling by using the test path patterns specified in the Polygon package.
• Added a check to reject Polygon interactors, with a clear error message.
• Imported HEADER_EXTS and LANGUAGES from cms.grading.languagemanager to support the above enhancements.
• Updated the patch to improve checker output for partial scores, ensure proper message formatting, and clarify that interactors are not supported. Argument order for CMS is enforced, and checker exit codes and messages are improved for CMS integration.

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[codecov]

Codecov Report

❌ Patch coverage is 3.50877% with 55 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.62%. Comparing base (cc9dfaf) to head (36cf2ad).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
cmscontrib/loaders/polygon.py	3.50%	55 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1725      +/-   ##
==========================================
- Coverage   56.72%   56.62%   -0.11%     
==========================================
  Files         344      344              
  Lines       28307    28360      +53     
==========================================
+ Hits        16057    16058       +1     
- Misses      12250    12302      +52     

Flag	Coverage Δ
functionaltests	0.00% <0.00%> (ø)
unittests	56.62% <3.50%> (-0.11%)	⬇️

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