Add js_repl kernel crash diagnostics

[fjord-oai]

Summary

This PR improves js_repl crash diagnostics so kernel failures are debuggable without weakening timeout/reset guarantees.

What Changed

• Added bounded kernel stderr capture and truncation logic (line + byte caps).
• Added structured kernel snapshots (pid, exit status, stderr tail) for failure paths.
• Enriched model-visible kernel-failure errors with a structured diagnostics payload:
  • js_repl diagnostics: {...}
  • Included only for likely kernel-failure write/EOF cases.
• Improved logging around kernel write failures, unexpected exits, and kill/wait paths.
• Added/updated unit tests for:
  • UTF-8-safe truncation
  • stderr tail bounds
  • structured diagnostics shape/truncation
  • conditional diagnostics emission
  • timeout kill behavior
  • forced kernel-failure diagnostics

Why

Before this, failures like broken pipe / unexpected kernel exit often surfaced as generic errors with little context. This change preserves existing behavior but adds actionable diagnostics while keeping output bounded.

Scope

• Code changes are limited to:
  • /Users/fjord/code/codex-jsrepl-seq/codex-rs/core/src/tools/js_repl/mod.rs

Validation

• cargo clippy -p codex-core --all-targets -- -D warnings
• Targeted codex-core js_repl unit tests (including new diagnostics/timeout coverage)
• Tried starting a long running js_repl command (sleep for 10 minutes), verified error output was as expected after killing the node process.

git stack

• 👉 1 Add js_repl kernel crash diagnostics #11666
• ⏳ 2 Add feature-gated js_repl polling flow #10673
• ⏳ 3 Add optional js_repl isolation and vendored Node runtime #10670

[aaronl-openai]

thank you!