fix(vector_stores): normalize scores to similarity (higher = better) across all backends (#5391)

tests/vector_stores/test_supabase.py

@@ -75,7 +75,7 @@ def test_search_vectors(supabase_instance, mock_collection):
 
     assert len(results) == 2
     assert results[0].id == "id1"
-    assert results[0].score == 0.9
+    assert results[0].score == pytest.approx(0.1)
     assert results[0].payload == {"name": "vector1"}