Add integration test for regression task split integrity (Task 2280)

[hxrshxz]

Metadata

Item	Value
Reference Issue	NA
New Tests Added	Yes
Documentation Updated	No
Change Log Entry	Add integration test for regression task split integrity (Task 2280)

Description

What does this PR implement?

• Adds a new integration test tests/test_tasks/test_regression_task_splits.py.
• The test validates the split logic of a real regression task (OpenML task 2280):
  1. Confirms the task type is SUPERVISED_REGRESSION.
  2. Checks that the task uses 10‑fold cross‑validation with a single repeat.
  3. Verifies that train and test indices are NumPy arrays, non‑overlapping, and within dataset bounds.
  4. Ensures the union of all test indices covers the entire dataset.

Why is this change necessary?

• Existing tests cover classification and clustering tasks but lack regression split verification.
• Correct split handling is essential for reproducible experiments and for downstream UI components that consume OpenML data.

How to run the test

# From the repository root (virtual environment activated)
pytest tests/test_tasks/test_regression_task_splits.py