When open_or_rebuild_index is called and the index directory does not exist,
return a fresh in-memory Tantivy index instead of creating the directory as
a side effect. This prevents workspace contamination during test runs where
INDEX_DIR has not been redirected to a temp directory.
In production the data directory is always created during setup, so disk-
based indexes continue to work normally.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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