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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