(fix): check xarray installation when using any xarray-dependent code

[ilan-gold]

• Closes: QOL issue
• Tests added
• Release note added (or unnecessary)

[codecov]

Codecov Report

Attention: Patch coverage is 98.76543% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.34%. Comparing base (cbdbb14) to head (e770b69).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
src/anndata/compat/__init__.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1991      +/-   ##
==========================================
- Coverage   87.11%   85.34%   -1.78%     
==========================================
  Files          46       46              
  Lines        6947     7001      +54     
==========================================
- Hits         6052     5975      -77     
- Misses        895     1026     +131     

Files with missing lines	Coverage Δ
src/anndata/_core/merge.py	85.06% <100.00%> (-8.89%)	⬇️
src/anndata/_core/xarray.py	98.43% <100.00%> (+0.81%)	⬆️
src/anndata/_io/specs/lazy_methods.py	96.12% <100.00%> (+0.07%)	⬆️
src/anndata/_io/zarr.py	84.78% <100.00%> (ø)
src/anndata/experimental/backed/_io.py	96.15% <100.00%> (+5.24%)	⬆️
src/anndata/tests/helpers.py	82.91% <100.00%> (-10.24%)	⬇️
src/anndata/compat/__init__.py	80.64% <75.00%> (+4.11%)	⬆️

... and 5 files with indirect coverage changes

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[ilan-gold]

The codecov is complaining because we need a similar fix to scverse/scanpy#3607 (comment) where we have "min-deps" i.e., no xarray dask etc.

[flying-sheep]

Yeah! Would you do the equivalent PR to scverse/scanpy#3607 or should I?

[ilan-gold]

I'm happy to do it @flying-sheep.

[ilan-gold]

dev was installing test which was installing xarray, so I removed it. I couldn't see a reason to have it, but it's possible there is some sort of unknown use-case for it (to me)

[ilan-gold]

So this was also a pretty big whif on our test suite since we weren't testing without xarray

[ilan-gold]

The awkward array docs are down (?) so we'll just rerun tomorrow