fix(app-server): speed up shutdown

[fcoury-oai]

Why

Pressing Ctrl+C or Ctrl+D in the TUI could make Codex pause during shutdown when app-server background work still held outbound sender clones.

Shutdown tracing against the current ~/.codex path found three relevant holders:

• SkillsWatcher kept its event-loop task alive until the shutdown timeout path.
• AppServerAttestationProvider retained a strong Arc<OutgoingMessageSender>, which could keep outbound teardown waiting after the processor task had exited.
• A background apps/list task could still own an outbound sender when shutdown began, causing the in-process app-server runtime to wait for its outbound channel to close.

What Changed

• Give SkillsWatcher an explicit shutdown CancellationToken and cancel it from app-server teardown so its event loop drops the outbound sender promptly.
• Change AppServerAttestationProvider to keep a Weak<OutgoingMessageSender> and return immediately when it can no longer be upgraded.
• Give AppsRequestProcessor a shutdown CancellationToken and cancel in-flight background apps/list work during teardown.

How to Test

1. Start Codex TUI from a real home configuration.
2. Press Ctrl+C.
3. Confirm Codex exits promptly instead of pausing during shutdown.
4. Repeat with Ctrl+D and confirm the same prompt exit path.

Focused manual trace validation from the investigation:

• Before the full fix, reproduced shutdown traces showed outbound teardown waiting on lingering owners, including attestation.provider=1 and later apps.list.task=1.
• After the fix, fresh real-home Ctrl+D traces showed app_server.runtime.outbound_state_after_processor_join with owners=none, app_server.runtime.wait_outbound_handle = 0ms, and total TUI app-server shutdown around 18ms.

Targeted validation:

• RUST_MIN_STACK=8388608 cargo test -p codex-app-server