feat(client/stdio): opt-in process-tree teardown on close()

[siddheshbandgar]

Fixes #2023.

Problem

StdioClientTransport.close() signals only the direct child. When a server is launched through a wrapper — npx, uvx, python -m — the wrapper dies and the real server is orphaned. On Windows ChildProcess.kill() cannot terminate a tree at all.

Change

Adds an opt-in killProcessTree option to StdioServerParameters (Options A/B from the issue, combined):

• POSIX: the child is spawned with detached: true so it leads its own process group, and close() signals -pid, reaching every descendant.
• Windows: teardown goes through taskkill /pid <pid> /T /F.
• Both paths fall back to the existing proc.kill(signal) if the group/taskkill call fails.

Default is false, so signal propagation and Ctrl-C behaviour are unchanged for anyone who doesn't opt in — this is deliberate, since detached: true by default would turn a crashed host into a permanent orphan factory (the trade-off called out as Option B in the issue).

Test

stdioKillProcessTree.test.ts spawns a wrapper that forks a long-lived grandchild, records its pid, closes the transport, and asserts the grandchild is reaped. Skipped on Windows in CI; the taskkill path was verified manually.

Happy to change the default, or move to Option C (a consumer-supplied cleanup hook), if you'd prefer a different direction.

A changeset is included (client: minor).

[changeset-bot]

🦋 Changeset detected

Latest commit: cb22c83

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 10 packages

Name	Type
@modelcontextprotocol/client	Minor
@modelcontextprotocol/core	Minor
@modelcontextprotocol/server	Minor
@modelcontextprotocol/server-legacy	Minor
@modelcontextprotocol/codemod	Minor
@modelcontextprotocol/core-internal	Patch
@modelcontextprotocol/express	Major
@modelcontextprotocol/fastify	Major
@modelcontextprotocol/hono	Major
@modelcontextprotocol/node	Major

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commit: cb22c83