docs: clarify in-place string conversion during writes

[rastagan-git]

• Closes Side effect when writing string columns to zarr #2610

• Tests added

• Release note not necessary because: this clarifies existing API behavior without changing runtime behavior.

Summary

Clarify that convert_strings_to_categoricals=True modifies obs, var, and raw.var in place before write_h5ad() and write_zarr() serialize the object.

The updated parameter documentation also points users to convert_strings_to_categoricals=False when they need to preserve the in-memory dtypes.

Context

Both writers call the in-place AnnData.strings_to_categoricals() helper before writing. This behavior is longstanding, and #1474 made it optional, but the public method documentation currently says only “Convert string columns to categorical,” which does not make the side effect clear.

This PR documents the current contract only. It does not change the default, serialization behavior, or on-disk format.

Validation

• python -m pytest tests/test_readwrite.py::test_write_categorical -q — 6 passed
• ruff check src/anndata/_core/anndata.py
• ruff format --check src/anndata/_core/anndata.py
• git diff --check

AI assistance disclosure: I used Codex to help trace the write paths, inspect the history, and draft the documentation. I manually reproduced the behavior, reviewed the final diff, and ran the listed checks.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.84%. Comparing base (d2cc7ee) to head (0adcc4e).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2626      +/-   ##
==========================================
- Coverage   87.68%   85.84%   -1.84%     
==========================================
  Files          49       49              
  Lines        8102     8102              
==========================================
- Hits         7104     6955     -149     
- Misses        998     1147     +149     

Files with missing lines	Coverage Δ
src/anndata/_core/anndata.py	86.40% <ø> (ø)

... and 8 files with indirect coverage changes