[codex] Add exec-server SIGTERM shutdown

[richardopenai]

Summary

• Add Unix SIGTERM handling to codex exec-server so the service stops accepting new WebSocket connections and shuts down cleanly.
• Thread a cancellation token through active connection processing so in-flight reads and route handling can exit promptly.
• Drain all exec-server sessions during process shutdown and terminate owned child processes through the existing process shutdown path.

Impact

This makes standalone exec-server behave like a long-lived service when a supervisor or user sends SIGTERM, instead of relying on abrupt process termination.

Validation

• just fmt
• cargo test -p codex-exec-server --test process exec_server_sigterm_shuts_down_gracefully
• cargo test -p codex-exec-server --test process
• cargo test -p codex-exec-server
• just fix -p codex-exec-server
• Manual local end-to-end test against target/debug/codex exec-server over raw WebSocket: initialize, start a long-running shell process, send SIGTERM to exec-server, verify exit code 0, WebSocket disconnect, and child process cleanup.

Notes

• just bazel-lock-update and just bazel-lock-check could not run locally because bazel is not installed in this environment.

[chatgpt-codex-connector]

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