Fix 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 85.61%. Comparing base (829abb6) to head (d16a246).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2460   +/-   ##
=======================================
  Coverage   85.60%   85.61%           
=======================================
  Files          49       49           
  Lines        7671     7673    +2     
=======================================
+ Hits         6567     6569    +2     
  Misses       1104     1104           

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