chore: Clean up rag memory dependencies

PiperOrigin-RevId: 823556599

Author: DeanChensj

src/google/adk/dependencies/vertexai.py

@@ -15,3 +15,4 @@
 from __future__ import annotations
 
 import vertexai
+from vertexai.preview import rag

src/google/adk/memory/vertex_ai_rag_memory_service.py

@@ -24,9 +24,9 @@
 
 from google.genai import types
 from typing_extensions import override
-from vertexai.preview import rag
 
 from . import _utils
+from ..dependencies.vertexai import rag
 from .base_memory_service import BaseMemoryService
 from .base_memory_service import SearchMemoryResponse
 from .memory_entry import MemoryEntry