StdioServerParameters exposes no preexec_fn / rlimit / process-group hook for spawned MCP server subprocess

Context

StdioServerParameters (and the stdio_client context manager that spawns the server subprocess) expose no hook to control the spawned child process's resource limits or process-group assignment at fork time.

Problem

A client that spawns an untrusted or third-party MCP server via stdio cannot, from the SDK public API:

• set CPU / memory / FD / address-space rlimits on the child,
• place the child in its own process group (so a runaway child can be killed as a group without orphaning grandchildren),
• set a preexec_fn (POSIX) or equivalent to run arbitrary setup between fork and exec.

StdioServerParameters signature (command, args, env, cwd, encoding, encoding_error_handler) has no slot for any of these. stdio_client owns the subprocess spawn internally, so a caller cannot inject a custom Popen either.

Impact

Downstream hosts (e.g. inference servers spawning MCP tool servers) cannot enforce hard resource caps or reliable teardown on a hung/misbehaving MCP server subprocess from the client side. The only mitigation available today is a bounded connect timeout around __aenter__, which does not cover a server that accepts the connection then later runs away.

Request

Expose at least one of:

1. an optional preexec_fn / process_group / rlimit-style kwarg on StdioServerParameters (or stdio_client), passed through to the underlying subprocess.Popen, or
2. an injection point for a custom Popen factory / spawn callable.

(1) mirrors subprocess.Popen(..., preexec_fn=..., start_new_session=...) and would let hosts enforce resource limits + process-group isolation without forking the SDK.

Environment: mcp python-sdk, macOS / Linux. Filed from fusion-mlx (local MLX inference host) where we need to cap spawned MCP server subprocesses.

Beginne damit, StdioServerParameters und den stdio_client-Kontextmanager zu lokalisieren. Verfolge anschließend, wie sie den subprocess erstellen und welche Optionen von subprocess.Popen derzeit übergeben werden. Vergleiche die angeforderten pre-exec-, process-group-, resource-limit- oder spawn-factory-Ansätze unter macOS und Linux. Die Aufgabe ist abgeschlossen, wenn die öffentliche API einen dokumentierten process-control-Hook bereitstellt und sein Verhalten durch die relevanten Tests abgedeckt ist.