feat: remove sparse data scipy inheritance

[ilan-gold]

• Closes Refactor BaseCompressedSparseDataset and subclasses to remove backed_{csr,csc}_matrix #1456
• Tests added
• Release note added (or unnecessary)

[codecov]

Codecov Report

❌ Patch coverage is 92.02454% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.50%. Comparing base (a892252) to head (7e7c4ac).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/anndata/_core/sparse_dataset.py	91.87%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1927      +/-   ##
==========================================
- Coverage   86.72%   84.50%   -2.23%     
==========================================
  Files          46       46              
  Lines        7196     7177      -19     
==========================================
- Hits         6241     6065     -176     
- Misses        955     1112     +157     

Files with missing lines	Coverage Δ
src/anndata/_core/anndata.py	83.33% <100.00%> (+0.14%)	⬆️
src/anndata/compat/__init__.py	81.93% <100.00%> (-3.73%)	⬇️
src/anndata/_core/sparse_dataset.py	89.57% <91.87%> (-3.23%)	⬇️

... and 6 files with indirect coverage changes