Always write indptr as int64

[ivirshup]

• Closes Sparse matrices written with 32bit indptr #1261
• Tests added
• Release note added (or unnecessary)

Opening as a draft to show how many tests fail

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (299ca97) 85.65% compared to head (4f56751) 83.36%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1262      +/-   ##
==========================================
- Coverage   85.65%   83.36%   -2.29%     
==========================================
  Files          34       34              
  Lines        5460     5460              
==========================================
- Hits         4677     4552     -125     
- Misses        783      908     +125     

Flag	Coverage Δ
gpu-tests	?

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Files	Coverage Δ
anndata/_io/specs/methods.py	87.92% <100.00%> (-0.23%)	⬇️

... and 7 files with indirect coverage changes