chore: upgrade rerank model to v4.0-fast

- Updated default model from rerank-v3.5 to rerank-v4.0-fast
- v4.0 is Cohere's latest reranking model (Dec 2024)
- Supports 100+ languages with better accuracy
- Alternative: rerank-v4.0-pro for max accuracy

Author: DevanshuNEU

backend/services/search_v2/hybrid_searcher.py

@@ -28,7 +28,7 @@ def __init__(
         pinecone_index,
         embedding_fn,
         cohere_api_key: Optional[str] = None,
-        rerank_model: str = "rerank-v3.5",
+        rerank_model: str = "rerank-v4.0-fast",
     ):
         self.index = pinecone_index
         self.embed = embedding_fn