js_repl: block wrapped payload prefixes in grammar

[fjord-oai]

Summary

Tighten the js_repl freeform Lark grammar to block the most common malformed payload wrappers before they reach runtime validation.

What Changed

• Replaced the overly permissive js_repl freeform grammar (start: /[\s\S]*/) with a structured grammar that still supports:
  • plain JS source
  • optional first-line // codex-js-repl: pragma followed by JS source
• Added grammar-level filtering for common bad payload shapes by rejecting inputs whose first significant token starts with:
  • { (JSON object wrapper like {"code":"..."})
  • " (quoted code string)
  • ``` (markdown code fences)
• Implemented the grammar without regex lookahead/lookbehind because the API-side Lark regex engine does not support look-around.
• Added a unit test to validate the grammar shape and guard against reintroducing unsupported lookaround.

Why

js_repl is a freeform tool, but the model sometimes emits wrapped payloads (JSON, quoted strings, markdown fences) instead of raw JavaScript. We already reject those at runtime, but this change moves the constraint into the tool grammar so the model is less likely to generate invalid tool-call payloads in the first place.

Testing

• cargo test -p codex-core js_repl_freeform_grammar_blocks_common_non_js_prefixes
• cargo test -p codex-core parse_freeform_args_rejects_

Notes

• This intentionally over-blocks a few uncommon valid JS starts (for example top-level { ... } blocks or top-level quoted directives like "use strict";) in exchange for preventing the common wrapped-payload mistakes.

git stack

• 👉 1 js_repl: block wrapped payload prefixes in grammar #12300
• ⏳ 2 js_repl: remove codex.state helper references #12275
• ⏳ 3 ci(bazel): install Node from node-version.txt in remote image #12205
• ⏳ 4 tests(js_repl): remove node-related skip paths from js_repl tests #12185
• ⏳ 5 Add feature-gated js_repl polling flow #10673