Unify split logic in WithinSessionEvaluation

- Remove manual session loops from _evaluate and _evaluate_learning_curve
- Use WithinSessionSplitter to handle session iteration internally
- _evaluate_learning_curve now uses WithinSessionSplitter with
  StratifiedShuffleSplit as inner CV class for consistency
- Track session changes from splitter indices and accumulate scores per session
- Reset random state per session in WithinSessionSplitter to match
  original evaluation behavior where new splitters were created per session
- Update test reference to match this behavior
- Use random_state in learning curve evaluation for reproducibility

Author: bruAristimunha