Limit contest dump scope

[pxsit]

Prevent exporting shared users and teams from pulling in participations from unrelated contests. The importer now traverses only objects rooted in selected contests, filtering unrelated references from older dumps.

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[codecov]

Codecov Report

❌ Patch coverage is 85.29412% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.75%. Comparing base (3277ca5) to head (0ef4405).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
cmscontrib/DumpImporter.py	84.37%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1730      +/-   ##
==========================================
+ Coverage   56.72%   56.75%   +0.03%     
==========================================
  Files         344      344              
  Lines       28314    28346      +32     
==========================================
+ Hits        16060    16088      +28     
- Misses      12254    12258       +4     

Flag	Coverage Δ
functionaltests	0.00% <0.00%> (ø)
unittests	56.75% <85.29%> (+0.03%)	⬆️

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