feat: 收敛知识库检索调度与评分语义

- 固定 rag.engine 与 Milvus 配置，补齐启动期检索基础设施校验

- 支持调用方配置 retrievalMode，并统一知识库检索入口与结果来源展示

- 修正关键词检索 knowledgeId 过滤、混合检索评分归一化与本地 ES 默认配置

easyflow-ui-admin/app/src/locales/langs/en-US/documentCollectionSearch.json

@@ -11,6 +11,7 @@
   "button": {
     "save": "Save Configuration"
   },
+  "engineHint": "The keyword search engine is controlled by platform-level configuration and is no longer configured per knowledge base.",
   "message": {
     "saveSuccess": "Configuration saved successfully",
     "saveFailed": "Configuration saved failed"