chore: allow implementations to skip defining a close method on Toolset

PiperOrigin-RevId: 792760747

Author: google-genai-bot

src/google/adk/tools/base_toolset.py

@@ -77,7 +77,6 @@ async def get_tools(
       list[BaseTool]: A list of tools available under the specified context.
     """
 
-  @abstractmethod
   async def close(self) -> None:
     """Performs cleanup and releases resources held by the toolset.