[Lint] Add python/sglang to ruff F401 checks and remove unused imports in files (#11685)

python/sglang/srt/entrypoints/context.py

@@ -1,6 +1,5 @@
 # SPDX-License-Identifier: Apache-2.0
 # Copied from vLLM
-import json
 import logging
 from abc import ABC, abstractmethod
 from typing import Union