ci: Add darts to downstream tests

[FBruzzesi]

What type of PR is this? (check all applicable)

• 💾 Refactor
• ✨ Feature
• 🐛 Bug Fix
• 🔧 Optimization
• 📝 Documentation
• ✅ Test
• 🐳 Other

Related issues

darts#2661 just got merged

Checklist

• Code follows style guide (ruff)
• Tests added
• Documented the changes

If you have comments or can explain your changes, please do so below

Darts entire test suite takes a bit too much for our use case - Narwhals is used in TimeSeries class, that's what I am adding as test here

[FBruzzesi]

Hey @dennisbader and @authierj, we try to run test suites for downstream libraries that use narwhals in our CI, to make sure that we don't break anything by accident.

As mentioned in the description, darts test suite takes ~30mins, but narwhals is used just in TimeSeries test. Do you think that's enough for now to test?

[dennisbader]

Hi @FBruzzesi, that's very nice 🚀

I'd also add the following tests (they're very lightweight):

• test_timeseries_multivariate.py, contains narwhals related checks for multivariate DataFrames.
• test_static_covariates.py, contains narwhals related checks for DataFrames and static covariates.
• we'll be adding checks using polars as backend soon. Polars will only be an optional dependency and not be part of our core.txt requirements file (nor dev.txt). But it looks like you already install it for the tests, so all good 👍

[FBruzzesi]

I'd also add the following tests (they're very lightweight):

• test_timeseries_multivariate.py, contains narwhals related checks for multivariate DataFrames.
• test_static_covariates.py, contains narwhals related checks for DataFrames and static covariates.

Thanks @dennisbader , I just added these two tests as well

• we'll be adding checks using polars as backend soon. Polars will only be an optional dependency and not be part of our core.txt requirements file (nor dev.txt). But it looks like you already install it for the tests, so all good 👍

Actually we don't install polars if not needed - I added it now, with a TODO note to use darts specification when you are going to introduce it in CI/some requirements