Kill stdio MCP children when a dial is interrupted

[RhysSullivan]

Fixes#1631.

Cancelling an in-flight health check or tool discovery (HTTP 499 on app refresh, or the 15s discovery timeout) abandoned the MCP connect handshake without closing the transport. The spawned stdio child stayed alive indefinitely; for docker run -i --rm integrations that stranded one container per interrupted dial.

• connectClient now passes the Effect.tryPromise abort signal into client.connect. On interruption the SDK fails the handshake and closes the transport (stdin end, then SIGTERM/SIGKILL), so the child exits.
• discoverTools wraps acquire/use in uninterruptibleMask so an interrupt landing between the handshake succeeding and the onExit cleanup attaching can no longer leak the connection. Connect and listTools stay restored, so cancellation and the discovery timeout still land promptly.
• Regression tests spawn a real stdio fixture and assert child-process lifetime across the three paths: uninterrupted discovery, fiber interrupt mid-handshake, and the discovery timeout. Both interrupt tests fail on main.

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Status	Name	Latest Commit	Updated (UTC)
✅ Deployment successful! View logs	executor-cloud	73cdb9f	Aug 18 2026, 04:12 PM

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commit: 73cdb9f