Fix read-only error in NaN reorder test under pandas >=3

[Marius1311]

What

Fix test_reorder_categories_with_nan_values, which fails only in the py3.14-pre (PRE-RELEASE DEPENDENCIES) CI job with:

ValueError: assignment destination is read-only

Root cause

The test injected a NaN with an in-place write:

adata.obs.loc[adata.obs.index[0], "leiden"] = np.nan

The fixture (get_example_data) runs sc.pp.filter_cells, which subsets the AnnData; anndata leaves the resulting obs backing arrays read-only. pandas >= 3 strictly refuses in-place mutation of read-only arrays, so the assignment raises. (On pandas 2.x the in-place write silently copied, so it passed — hence this only surfaced once a pandas 3 pre-release entered the pre-release matrix.)

This is a real forward-compatibility issue with the test's mutation pattern, not a masking workaround. The package code itself is unaffected — reorder_categories reassigns whole columns via cat.set_categories.

Fix

Inject the NaN on a writable copy and reassign the whole column:

leiden = adata.obs["leiden"].map({"0": "B cells", "1": "T cells"}).astype("category")
leiden = leiden.copy()
leiden.iloc[0] = np.nan
adata.obs["leiden"] = leiden

Test intent (reorder must preserve NaNs) is unchanged.

Verification

Reproduced faithfully (AnnData + sc.pp.filter_cells + the assignment) on pandas 3.0.3: the old pattern raises assignment destination is read-only; the new pattern succeeds (nan_count == 1). CI on this PR exercises the real py3.14-pre job.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.75%. Comparing base (a8872da) to head (65768e1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
- Coverage   80.83%   80.75%   -0.08%     
==========================================
  Files          15       15              
  Lines        1372     1372              
==========================================
- Hits         1109     1108       -1     
- Misses        263      264       +1     

see 1 file with indirect coverage changes

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