fix: add missing shutdown methods to PromptServiceImpl and ConversationServiceImpl (#3925)

Change is visible in server shutdown logs, changes `WARNING` loglines to
`INFO`

Signed-off-by: Nathan Weinberg <[email protected]>

Author: nathan-weinberg

src/llama_stack/core/conversations/conversations.py

@@ -312,3 +312,6 @@ async def openai_delete_conversation_item(
 
         logger.debug(f"Deleted item {item_id} from conversation {conversation_id}")
         return ConversationItemDeletedResource(id=item_id)
+
+    async def shutdown(self) -> None:
+        pass

src/llama_stack/core/prompts/prompts.py

@@ -230,3 +230,6 @@ async def set_default_version(self, prompt_id: str, version: int) -> Prompt:
         await self.kvstore.set(default_key, str(version))
 
         return self._deserialize_prompt(data)
+
+    async def shutdown(self) -> None:
+        pass