feat(rag): dense+BM25 hybrid on MilvusClientV2, drop SDK path

Replace legacy MilvusServiceClient knowledge search/write with a single
MilvusClientV2 hybrid store (BM25 function + dense ANN + RRFRanker).
Use collection biz_hybrid and require knowledge reindex.

src/main/java/com/superbiz/agent/constant/MilvusConstants.java

@@ -8,9 +8,14 @@ public class MilvusConstants {
     public static final String MILVUS_DB_NAME = "default";
     
     /**
-     * Milvus 集合名称
+     * Legacy collection name (pre hybrid). Knowledge path uses milvus.collection (default biz_hybrid).
      */
     public static final String MILVUS_COLLECTION_NAME = "biz";
+
+    /**
+     * Default hybrid dense+BM25 collection.
+     */
+    public static final String MILVUS_HYBRID_COLLECTION_NAME = "biz_hybrid";
     
     /**
      * 向量维度（豆包 embedding 模型的维度）