fix: load raw X when leaving backed mode

[ehsanestaji]

• Closes Setting adata.filename = None doesn't load .X to memory #623
• Tests added

• Release note not necessary because:

Summary

Setting adata.filename = None on a backed object already materialized adata.X, but backed objects read from disk could still lose adata.raw.X because raw._X stayed None after the backing file was closed.

This PR materializes raw.X before leaving backed mode, adds a regression test for the backed-read path, and includes a release note fragment.

Checks

• uv run pytest tests/test_backed_hdf5.py::test_return_to_memory_mode_loads_raw_X -q
• uv run pytest tests/test_backed_hdf5.py::test_return_to_memory_mode tests/test_backed_hdf5.py::test_return_to_memory_mode_loads_raw_X tests/test_backed_hdf5.py::test_backed_raw tests/test_raw.py::test_raw_backed -q
• uv run pytest tests/test_raw.py -q
• uv run pytest tests/test_backed_hdf5.py -q
• uv run ruff check src/anndata/_core/file_backing.py tests/test_backed_hdf5.py
• uv run ruff format --check src/anndata/_core/file_backing.py tests/test_backed_hdf5.py
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.87%. Comparing base (c37e213) to head (b5958cd).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2460      +/-   ##
==========================================
- Coverage   87.81%   85.87%   -1.95%     
==========================================
  Files          49       49              
  Lines        7799     7801       +2     
==========================================
- Hits         6849     6699     -150     
- Misses        950     1102     +152     

Files with missing lines	Coverage Δ
src/anndata/_core/file_backing.py	87.50% <100.00%> (+0.19%)	⬆️

... and 8 files with indirect coverage changes

[ehsanestaji]

Rebased onto current main and refreshed the PR head.

Local validation:

• uv run pytest tests/test_backed_hdf5.py tests/test_raw.py -q — 425 passed, 2 xfailed
• Ruff check and format check passed
• git diff --check passed