feat: 下沉知识库检索编排能力

- 新增 rag retrieval 核心协议、RRF 融合与相关度归一化

- 支持关键词检索按 knowledgeId 过滤并补充 ES/Lucene 单测

- 扩展 KnowledgeNode 检索模式与 Milvus 检索参数透传

easy-agents-store/easy-agents-store-milvus/src/main/java/com/easyagents/store/milvus/MilvusVectorStore.java

@@ -535,4 +535,15 @@ public class MilvusVectorStore extends DocumentStore {
     public MilvusClientV2 getClient() {
         return client;
     }
+
+    public boolean checkAvailable() {
+        try {
+            return client.hasCollection(HasCollectionReq.builder()
+                .collectionName("__milvus_boot_probe__")
+                .build()) != null;
+        } catch (Exception e) {
+            LOG.warn("Milvus availability check failed. message={}", e.getMessage());
+            return false;
+        }
+    }
 }