Adds fixture to download atlases for test_validation.py

[IgorTatarnikov]

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Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?
Tests were failing as test_validation.py tried to modify a directory that didn't exist.

https://github.com/brainglobe/brainglobe-atlasapi/actions/runs/13244823116/job/36975288009?pr=497

This failure seems to be related to the CI cache miss.

What does this PR do?
Ensures atlases are downloaded prior to test_validation.py running.

How has this PR been tested?

All tests pass locally and on CI.

Is this a breaking change?

No

Does this PR require an update to the documentation?

No

Checklist:

• The code has been tested locally
• The code has been formatted with pre-commit

[niksirbi]

Looks good Igor.

There's one thing I'm not sure about (because I don't know the working of atlasapi very well):
Here you are checking if 3 atlases of specific version exist, but upon download (BrainGlobeAtlas instantiation), you don't actually enforce the downloading of the specific versions you checked. I assume the latest version will be fetched instead. Will there be a problem if the specific versions you check are no longer the latest ones?

[IgorTatarnikov]

Thanks!

The code in test_validation.py also looks for specific atlas versions when looking in the directory tree, see: https://github.com/brainglobe/brainglobe-atlasapi/blob/4b5fa6479a0cf209a15f853cf48be6fa33f9bb6e/tests/atlasgen/test_validation.py#L32C1-L44C35.

I think that's something we should discuss at some point, do we run tests on specific versions or the latest? And enforce that as standard across the test suite.