XPC: reply to unknown routes and make the request timeout actually cancel the waiter

[radheradhe01]

Problem

Two related issues in the XPC request path:

1. Unknown route hangs the client. XPCServer dispatches with if let handler = routes[route] { ... } and no else. An unknown route produces no reply at all, so the client blocks until its timeout (or forever, if it sent none).

2. The client timeout is illusory. XPCClient.send races a timeout task against the reply task in a withThrowingTaskGroup. The reply task uses withCheckedThrowingContinuation, which is not cancellation-aware. When the timeout fires, the group tears down and awaits the reply task — which only completes when the daemon actually replies. Against a live-but-hung daemon the "timeout" never returns.

Fix

• XPCServer: add the else branch and reply with an invalidArgument error for unknown routes.
• XPCClient: wrap the reply task in withTaskCancellationHandler and bridge the continuation through a small resume-once box (XPCReplyBox). On cancellation the continuation is resumed promptly; a late XPC reply becomes a no-op. The box guarantees the continuation is resumed exactly once.

Notes

The connection is left intact on timeout; only the pending request is abandoned.