app-server: own thread listener teardown

[winston-openai]

Summary

• make the listener registry own the exact task abort handle together with its command sender
• publish listener state and its task handle atomically, and make stale cleanup identity-conditional
• abort and await registered listeners during app-server shutdown
• roll back only the exact thread state whose listener attachment failed

This is the first app-server layer of the thread-bound idle replacement stack and depends on #31350.

Validation

• CARGO_INCREMENTAL=0 just test -p codex-app-server thread_state::tests
• CARGO_INCREMENTAL=0 just test -p codex-app-server removing_thread_state_clears_listener_and_active_turn_history
• CARGO_INCREMENTAL=0 just test -p codex-app-server app_server_event_sink_uses_listener_fifo_for_goal_updates_and_clears

Stack

• core: preserve thread activity through completion #31350
• this PR
• teardown singleflight and lifecycle serialization follow-up
• complete idle-tree replacement follow-up
• semantic resume-config reuse follow-up

[winston-openai]

@codex review

[chatgpt-codex-connector]

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

Reviewed commit: 7d7829ea6b

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[winston-openai]

@codex review