fix: reallow h5 slash writing

[ilan-gold]

• Closes Request: Migration support for / character restriction in keys #2039
• Tests added
• Release note added (or unnecessary)

[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.56%. Comparing base (e2d7503) to head (fda5ad7).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/anndata/_io/zarr.py	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2097      +/-   ##
==========================================
- Coverage   85.57%   85.56%   -0.01%     
==========================================
  Files          46       46              
  Lines        7070     7080      +10     
==========================================
+ Hits         6050     6058       +8     
- Misses       1020     1022       +2     

Files with missing lines	Coverage Δ
src/anndata/_io/specs/registry.py	94.50% <100.00%> (+0.02%)	⬆️
src/anndata/_settings.py	94.33% <100.00%> (+0.03%)	⬆️
src/anndata/_io/zarr.py	82.22% <75.00%> (-0.71%)	⬇️

[flying-sheep]

looks great! don’t forget to add the issue tracking the removal to the next version’s milestone.