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

issue で参照されている errlog の定義がある src/mcp/client/stdio/init.py から始め、Python の subprocess.Popen と anyio.create_process で受け入れられる stderr の値を比較します。subprocess.DEVNULL の意図された扱いを確認し、それに応じて client API を更新します。呼び出し側が整数値を型や実行時の問題なしに渡せ、既存の stderr の動作が引き続きカバーされていれば完了です。