Summary

Improve js_repl behavior when the Node kernel hits a process-level failure (for example, an uncaught exception or unhandled Promise rejection).

Instead of only surfacing a generic js_repl kernel exited unexpectedly after stdout EOF, js_repl now returns a clearer exec error for the active request, then resets the kernel cleanly.

Why

Some sandbox-denied operations can trigger Node errors that become process-level failures (for example, an unhandled EventEmitter 'error' event). In that case:

• the kernel process exits,
• the host sees stdout EOF,
• the user gets a generic kernel-exit error,
• and the next request can briefly race with stale kernel state.

This change improves that failure mode without monkeypatching Node APIs.

Changes

Kernel-side (js_repl Node process)

• Add process-level handlers for:
  • uncaughtException
  • unhandledRejection
• When one of these fires:
  • best-effort emit a normal exec_result error for the active exec
  • include actionable guidance to catch/handle async errors (including Promise rejections and EventEmitter 'error' events)
  • exit intentionally so the host can reset/restart the kernel

Host-side (JsReplManager)

• Clear dead kernel state as soon as the stdout reader observes unexpected kernel exit/EOF.
• This lets the next js_repl exec start a fresh kernel instead of hitting a stale broken-pipe path.

Tests

• Add regression coverage for:
  • uncaught async exception -> exec error + kernel recovery on next exec
• Update forced-kernel-exit test to validate recovery behavior (next exec restarts cleanly)

Impact

• Better user-facing error for kernel crashes caused by uncaught/unhandled async failures.
• Cleaner recovery behavior after kernel exit.

Validation

• cargo test -p codex-core --lib tools::js_repl::tests::js_repl_uncaught_exception_returns_exec_error_and_recovers -- --exact
• cargo test -p codex-core --lib tools::js_repl::tests::js_repl_forced_kernel_exit_recovers_on_next_exec -- --exact
• just fmt