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

Inizia in src/mcp/client/stdio/init.py, alla definizione di errlog citata nell’issue, quindi confronta i valori di stderr accettati in Python subprocess.Popen e anyio.create_process. Conferma la gestione prevista di subprocess.DEVNULL e aggiorna di conseguenza l’API client; il lavoro è completato quando i chiamanti possono passare il valore intero senza problemi di tipo o di runtime e il comportamento esistente di stderr rimane coperto.