test(backed): add regression for filename=None after read_h5ad

[SID-6921]

Summary

Adds regression coverage for switching from backed mode to memory mode when loading with
ead_h5ad(..., backed='r+') and then setting �data.filename = None.

Details

• Adds a new parametrized test covering dense, CSR, and CSC X payloads.
• Verifies that after ilename=None:
  • isbacked is False
  • X is not None
  • data values are preserved

Validation

• python -m pytest tests/test_backed_hdf5.py -k return_to_memory_mode -q

Refs #623

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.71%. Comparing base (0569b0c) to head (00c4376).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2517      +/-   ##
==========================================
- Coverage   87.66%   85.71%   -1.96%     
==========================================
  Files          49       49              
  Lines        7729     7729              
==========================================
- Hits         6776     6625     -151     
- Misses        953     1104     +151     

see 8 files with indirect coverage changes

[SID-6921]

I pushed a follow-up commit to satisfy pre-commit linting on the new regression test. Current failing checks are triage gates (Triage: Check PR title, milestone, and labels and Triage: Check if GPU tests are allowed to run), which I can’t resolve from a fork. Could a maintainer please help set the required labels/milestone and triage permissions? Thanks!