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

從 issue 中引用的 src/mcp/client/stdio/init.py 內 errlog 定義開始，然後比較 Python subprocess.Popen 和 anyio.create_process 接受的 stderr 值。確認對 subprocess.DEVNULL 的預期處理方式，並相應更新 client API；當呼叫端可以傳入整數值而不會出現型別或執行階段問題，且現有的 stderr 行為仍獲得涵蓋時，即視為完成。