Tests that create or consume documents trigger the search index signal handler,
which calls get_backend().add_or_update() against settings.INDEX_DIR. This
class only inherited SampleDirMixin, leaving INDEX_DIR pointing at the default
non-existent path and causing FileNotFoundError in CI.
Added _search_index fixture to documents/tests/conftest.py: creates a temp
index directory, overrides INDEX_DIR, and resets the backend singleton.
Applied via @pytest.mark.usefixtures on the class.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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