refactor: extract UI test socket sanity coordinator

[teamleaderleo]

Problem

AppDelegate contained two copies of UI-test socket probing: diagnostics payload generation and the delayed sanity/restart sequence. The extraction history also exposed invalid return payload statements in two void helpers.

Change

Extract the probe and restart lifecycle into UITestSocketSanityCoordinator. It preserves the exact diagnostics keys, synchronous probe behavior, restart source, and 750 ms ordering delays. New scheduling uses cancellable ContinuousClock tasks. The existing void helpers are corrected to return normally when disabled.

This PR is stacked on #73.

Validation

• swiftc -D DEBUG -parse Sources/Debug/UITests/UITestSocketSanityCoordinator.swift Sources/AppDelegate.swift
• git diff --check

A full xcodebuild remains blocked by the fork worktree's missing vendor/bonsplit/Package.swift dependency.

[teamleaderleo]

Consolidated into the coherent upstream slices #78 (UI-test infrastructure), #79 (session lifecycle), #80 (window ownership), and #82 (terminal diagnostics). The original branch remains available for history.