fix(proxy): import assert_never and Required from typing_extensions

Three more unguarded 3.11-only stdlib imports landed while this PR was open, in
tool_search.py, agent_endpoints/endpoints.py and gemini_audio_transcription.py.
Each one breaks `import litellm` on Python 3.10 again, and the gate this PR adds
catches all three, which is the drift the issue describes.

litellm/proxy/_experimental/mcp_server/tool_search.py

@@ -3,9 +3,9 @@ from __future__ import annotations
 import json
 from collections.abc import Mapping, Sequence
 from datetime import datetime
-from typing import TYPE_CHECKING, Any, Final, TypedDict, assert_never
+from typing import TYPE_CHECKING, Any, Final, TypedDict
 
-from typing_extensions import ReadOnly, Required
+from typing_extensions import ReadOnly, Required, assert_never
 
 import litellm
 from litellm.proxy.agent_endpoints.agent_search import DEFAULT_AGENT_SEARCH_TOP_K

litellm/proxy/agent_endpoints/endpoints.py

@@ -13,10 +13,10 @@ import os
 import uuid
 from collections.abc import Mapping, Sequence
 from types import MappingProxyType
-from typing import Annotated, Final, TypedDict, assert_never
+from typing import Annotated, Final, TypedDict
 
 from fastapi import APIRouter, Depends, HTTPException, Query, Request
-from typing_extensions import ReadOnly, Required
+from typing_extensions import ReadOnly, Required, assert_never
 
 import litellm
 from litellm._logging import verbose_proxy_logger

litellm/types/llms/gemini_audio_transcription.py

@@ -1,7 +1,7 @@
-from typing import Literal, Required
+from typing import Literal
 
 from pydantic import BaseModel, ConfigDict
-from typing_extensions import ReadOnly, TypedDict
+from typing_extensions import ReadOnly, Required, TypedDict
 
 
 class GeminiTranscriptionAudioInput(TypedDict):