app-server: clear stale thread watches after v2 agent interruption

[jif-oai]

Why

PR #27007 moved MultiAgentV2 interruption reporting from the legacy collaboration close event to SubAgentActivity::Interrupted. App-server's missing-thread cleanup still ran only for the legacy event, so an interrupted child that had already been unloaded could remain marked as loaded and running in ThreadWatchManager. That leaves thread status and running-turn accounting stale, including the count used during graceful shutdown.

What changed

• Handle SubAgentActivity::Interrupted separately in app-server event processing.
• Remove the child's thread watch when ThreadManager no longer has that thread.
• Continue forwarding the same completed sub-agent activity notification to clients.

Testing

• Added a regression test that starts with a running watch for an unloaded child, applies the interrupted activity event, and verifies the watch is removed, the running count returns to zero, and the client notification is still emitted.

[jif-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Hooray!

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