refactor: remove unused codes, move core/agent module into dataset retrieval feature (#2614)

api/core/entities/agent_entities.py

@@ -0,0 +1,8 @@
+from enum import Enum
+
+
+class PlanningStrategy(Enum):
+    ROUTER = 'router'
+    REACT_ROUTER = 'react_router'
+    REACT = 'react'
+    FUNCTION_CALL = 'function_call'