fix: ensure radius values stored in AnnData.uns are writable

[selmanozleyen]

Closes #1247

RadiusBuilder and DelaunayBuilder stored an interval radius in
adata.uns['spatial_neighbors']['params'] as the tuple it was passed as.
anndata has no I/O spec for tuples, so any object built with a tuple radius
raised IORegistryError on write_h5ad / write_zarr.

Store a list instead, via a small _radius_to_uns helper. Scalar radii are
unchanged. Note that DelaunayBuilder already normalizes a scalar r to
(0.0, r) in __init__, so spatial_neighbors_delaunay(radius=5.0) now records
[0.0, 5.0] — the same value as before, just writable.

The abstract GraphBuilder.uns_params docstring now states that values must be
writable by anndata, so custom builders don't reintroduce the same problem.

Regression test covers both builders with scalar and interval radii, and asserts
on an actual write_h5ad rather than only on the stored value.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.55%. Comparing base (9b81fe7) to head (8e9fa54).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1249      +/-   ##
==========================================
+ Coverage   77.08%   78.55%   +1.47%     
==========================================
  Files          63       63              
  Lines        9436     9505      +69     
  Branches     1585     1586       +1     
==========================================
+ Hits         7274     7467     +193     
+ Misses       1561     1448     -113     
+ Partials      601      590      -11     

Files with missing lines	Coverage Δ
src/squidpy/gr/neighbors.py	91.66% <100.00%> (+0.87%)	⬆️

... and 3 files with indirect coverage changes

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[timtreis]

lgtm

[timtreis]

For clarification - we're doing this do stay backwards compatible? I'm wondering if we need this converter at all or if we should just fix it at creation time.