handles the niche case 🐩 in the hybrid search where:
1. a sort ranking rule is the first rule.
2. the keyword search is skipped at the first rule.
3. the semantic search is not skipped at the first rule.
Previously, we would have the skipped search winning, whereas we want the non skipped one winning.
- DistributionShift in Search object (to be set from model in embed?)
- Fix issue where embedder index wasn't computed at search time
- Accept as default embedder either the "default" one, or the only embedder when there is only one
