pydoctor Sphinx api-folder reproduction

This fixture reproduces twisted/pydoctor#728 and the missing test case requested on twisted/pydoctor#799. It demonstrates the failure when the pydoctor extension uses {outdir}/api but the Sphinx source tree does not generate an api/ subtree of its own.

Run it in an isolated environment:

python3.12 -m venv .venv
. .venv/bin/activate
python -m pip install -r requirements.txt
python reproduce.py

The oracle exits with code 23 and prints ORIGINAL_FAILURE to stderr only when both pydoctor and Sphinx produced their HTML artifacts before the extension failed to rename the absent api directory to api.sphinx_files.

The checked-in fixture was reduced with ReproMin. The oracle is intentionally more specific than a non-zero build exit: unrelated configuration, import, and documentation failures exit with code 1 instead of the expected code 23.