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 77.31%. Comparing base (ca07e64) to head (9bf4edd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1249      +/-   ##
==========================================
+ Coverage   77.24%   77.31%   +0.06%     
==========================================
  Files          63       63              
  Lines        9378     9380       +2     
  Branches     1579     1579              
==========================================
+ Hits         7244     7252       +8     
+ Misses       1532     1527       -5     
+ Partials      602      601       -1     

Files with missing lines	Coverage Δ
src/squidpy/gr/neighbors.py	91.73% <100.00%> (+0.94%)	⬆️

... and 1 file with indirect coverage changes

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