Add unit tests for check_values() and get_ax_idx()

[BhoomikaC15]

Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?
This PR increases the test coverage for functions that currently don't have any dedicated tests: check_values() in heatmaps.py and get_ax_idx() in slicer.py.

What does this PR do?
Adds total of 21 new tests in two new test files:

• test_check_values.py — 15 tests covering valid inputs (single/multiple regions, integers, floats, zeros), NaN handling (all NaN, partial NaN), and invalid inputs (None, strings, lists, unknown regions)
• test_get_ax_idx.py — 6 tests covering valid orientations (frontal, sagittal, horizontal), case sensitivity, and invalid/empty values
  Also uses a mock atlas fixture so no atlas download or network access needed.

References

Relates to issue #23

How has this PR been tested?

All 21 tests pass locally with pytest tests/test_unit/ -v on Python 3.13.5 / Windows.

Please explain how any new code has been tested, and how you have ensured that no existing functionality has changed.

Is this a breaking change?

No. This PR only adds new test files.

Does this PR require an update to the documentation?

No

Checklist:

• The code has been tested locally
• Tests have been added to cover all new functionality (unit & integration)
• The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit