More control over stderr in stdio client

Description

Currently, a TextIO object (defaulting to sys.stderr) must be set for errlog when creating a new stdio client session.
https://github.com/modelcontextprotocol/python-sdk/blob/2aa1ad2a69b56bc6cef089c9b4c3ecccbbc8e84d/src/mcp/client/stdio/__init__.py#L106
However, the underlying calls to Python's native Popen and anyio.create_process both support subprocess.DEVNULL: int = -3 for stderr, which allows the stream to be ignored.
By updating the type hint, we can easily support int for errlog. This change would benefit downstream projects and resolve issues for CLI users. Furthermore, I believe ignoring errlog is a better default than piping them into stderr, as the current behaviour can create unexpected side-effects that are difficult to trace.

References

Related Issues:
https://github.com/modelcontextprotocol/python-sdk/issues/156
https://github.com/langchain-ai/langchain-mcp-adapters/issues/72

Documentation:
https://docs.python.org/3/library/subprocess.html#subprocess.Popen
https://anyio.readthedocs.io/en/stable/api.html#anyio.open_process

从 issue 中引用的 src/mcp/client/stdio/init.py 内 errlog 定义开始，然后比较 Python subprocess.Popen 和 anyio.create_process 接受的 stderr 值。确认对 subprocess.DEVNULL 的预期处理方式，并相应更新 client API；当调用方可以传入整数值而不会出现类型或运行时问题，并且现有的 stderr 行为仍得到覆盖时，即视为完成。