Improve error message for inconsistent fold counts in OpenMLSplit

[axif0]

Replaced the generic any() check with an explicit loop to identify the mismatched repetition.
Implemented a descriptive error message that specifies which repetition has the incorrect number of folds and compares it to the expected count (from repetition 0).

[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 56.42%. Comparing base (4b1bdf4) to head (91e5abf).

Files with missing lines	Patch %	Lines
openml/tasks/split.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #1512       +/-   ##
============================================
- Coverage    79.90%   56.42%   -23.48%     
============================================
  Files           36       36               
  Lines         4349     4351        +2     
============================================
- Hits          3475     2455     -1020     
- Misses         874     1896     +1022     

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