Lesson 5 from the 4-day test-hell saga. The chroma cache lives at
tests/artifacts/.slop_cache/chroma_<collection>/, NOT at the per-run
live_gui_workspace_<timestamp>/ subdir. The trailing-slash bug in
Path(active_project_path).parent places the cache one level higher
than expected.
RAG tests must pre-clean the cache to avoid persistent state from
prior batched runs. Documents the cleanup pattern (shutil.rmtree with
ignore_errors=True), the auto-recovery mechanism (_validate_collection_dim),
and 3 anti-patterns (assuming per-run, not cleaning, asserting on
first chunk in batched context).
