chore(cleanup)!: remove tool_runtime.rag_tool (#3871)

Kill the `builtin::rag` tool group completely since it is no longer
targeted. We use the Responses implementation for knowledge_search which
uses the `openai_vector_stores` pathway.

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

llama_stack/apis/tools/tools.py

@@ -4,7 +4,6 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
-from enum import Enum
 from typing import Any, Literal, Protocol
 
 from pydantic import BaseModel
@@ -16,8 +15,6 @@ from llama_stack.apis.version import LLAMA_STACK_API_V1
 from llama_stack.providers.utils.telemetry.trace_protocol import trace_protocol
 from llama_stack.schema_utils import json_schema_type, webmethod
 
-from .rag_tool import RAGToolRuntime
-
 
 @json_schema_type
 class ToolDef(BaseModel):
@@ -181,22 +178,11 @@ class ToolGroups(Protocol):
         ...
 
 
-class SpecialToolGroup(Enum):
-    """Special tool groups with predefined functionality.
-
-    :cvar rag_tool: Retrieval-Augmented Generation tool group for document search and retrieval
-    """
-
-    rag_tool = "rag_tool"
-
-
 @runtime_checkable
 @trace_protocol
 class ToolRuntime(Protocol):
     tool_store: ToolStore | None = None
 
-    rag_tool: RAGToolRuntime | None = None
-
     # TODO: This needs to be renamed once OPEN API generator name conflict issue is fixed.
     @webmethod(route="/tool-runtime/list-tools", method="GET", level=LLAMA_STACK_API_V1)
     async def list_runtime_tools(