Fix DiskANN command dispatch when IVF is disabled

The command insert handler (used for runtime config like
search_list_size_search) was gated behind SQLITE_VEC_EXPERIMENTAL_IVF_ENABLE,
which defaults to 0. DiskANN commands were unreachable unless IVF was
also compiled in. Widen the guard to also activate when
SQLITE_VEC_ENABLE_DISKANN is set.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>