Round score at subtask level

[akmohtashami]

This change updates scoring to match the updated IOI rule: each subtask score is rounded to the task’s score_precision, and the final task score is computed as the sum of those rounded subtask scores (with a final rounding pass to avoid float artifacts).

It also allows us to avoid passing unrounded score around. I am keeping some rounding in ranking in case of float issues.

[akmohtashami]

FYI, @veluca93

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.16%. Comparing base (c682a55) to head (2589b1b).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
cms/server/contest/handlers/tasksubmission.py	0.00%	4 Missing ⚠️
...nit_tests/grading/scoretypes/GroupThresholdTest.py	75.00%	3 Missing ⚠️
...uite/unit_tests/grading/scoretypes/GroupMinTest.py	90.00%	2 Missing ⚠️
...uite/unit_tests/grading/scoretypes/GroupMulTest.py	90.00%	2 Missing ⚠️
cms/grading/scoretypes/__init__.py	0.00%	1 Missing ⚠️
cms/grading/scoretypes/abc.py	90.00%	1 Missing ⚠️
cms/server/admin/handlers/contestranking.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1690      +/-   ##
==========================================
- Coverage   54.17%   54.16%   -0.02%     
==========================================
  Files         340      340              
  Lines       27844    27835       -9     
==========================================
- Hits        15085    15077       -8     
+ Misses      12759    12758       -1     

Flag	Coverage Δ
functionaltests	0.00% <0.00%> (ø)
unittests	54.16% <81.81%> (-0.02%)	⬇️

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