The ANN vector queries with all-zero vectors are allowed even on vector indexes with similarity function set to cosine.
When enabling the rescoring option, those queries would fail as the rescoring calls `similarity_cosine` function underneath, causing an `InvalidRequest` exception as all-zero vectors were not allowed matching Cassandra's behaviour.
To eliminate the discrepancy we want the all-zero vector `similarity_cosine` calls to pass, but return the NaN as the cosine similarity for zero vectors is mathematically incorrect. We decided not to use arbitrary values contrary to USearch, for which the distance (not to be confused with similarity) is defined as cos(0, 0) = 0, cos(0, x) = 1 while supporting the range of values [0, 2].
If we wanted to convert that to similarity, that would mean sim_cos(0, x) = 0.5, which does not support mathematical reasoning why that would be more similar than for example vectors marking obtuse angles.
It's safe to assume that all-zero vectors for cosine similarity shouldn't make any impact, therefore we return NaN and eliminate them from best results.
Adjusted the tests accordingly to check both proper Cassandra and Scylla's behaviour.
Fixes: SCYLLADB-456
Backport to 2026.1 needed, as it fixes the bug for ANN vector queries using rescoring introduced there.
Closesscylladb/scylladb#28609
* github.com:scylladb/scylladb:
test/vector_search: add reproducer for rescoring with zero vectors
vector_search: return NaN for similarity_cosine with all-zero vectors
c5dc086baf