feat(server): add host process watchdog to StdioServerTransport

[aptsalt]

Summary

• Adds clientProcessId option to StdioServerTransport that enables a watchdog timer
• Watchdog periodically checks if the host process is alive using signal 0 (process.kill(pid, 0))
• Self-terminates the transport when the host is gone, preventing orphaned server processes
• Configurable check interval via watchdogIntervalMs (default: 3s)
• New StdioServerTransportOptions interface with backwards-compatible constructor overloads
• Timer uses unref() so it doesn't prevent clean process exit

Follows the same pattern used in vscode-languageserver-node as suggested in the issue.

Closes #208

Test plan

• should close transport when host process is gone — watchdog detects dead PID and closes
• should not close when host process is alive — no false positives with own PID
• should stop watchdog on close — cleanup on transport close
• should accept options object constructor — new constructor form works
• All 7 stdio tests pass
• Build succeeds

🤖 Generated with Claude Code

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🦋 Changeset detected

Latest commit: 25c2a3f

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

This PR includes changesets to release 4 packages

Name	Type
@modelcontextprotocol/server	Minor
@modelcontextprotocol/express	Major
@modelcontextprotocol/hono	Major
@modelcontextprotocol/node	Major

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