Persist js_repl codex helpers across cells (#14503)

## Summary

This changes `js_repl` so saved references to `codex.tool(...)` and
`codex.emitImage(...)` keep working across cells.

Previously, those helpers were recreated per exec and captured that
exec's `message.id`. If a persisted object or saved closure reused an
old helper in a later cell, the nested tool/image call could fail with
`js_repl exec context not found`.

This patch:
- keeps stable `codex.tool` and `codex.emitImage` helper identities in
the kernel
- resolves the current exec dynamically at call time using
`AsyncLocalStorage`
- adds regression coverage for persisted helper references across cells
- updates the js_repl docs and project-doc instructions to describe the
new behavior and its limits

## Why

We already support persistent top-level bindings across `js_repl` cells,
so persisted objects should be able to reuse `codex` helpers in later
active cells. The bug was that helper identity was exec-scoped, not
kernel-scoped.

Using `AsyncLocalStorage` fixes the cross-cell reuse case without
falling back to a single global active exec that could accidentally
attribute stale background callbacks to the wrong cell.

codex-rs/core/src/project_doc.rs

@@ -59,6 +59,7 @@ fn render_js_repl_instructions(config: &Config) -> Option<String> {
     );
     section.push_str("- `codex.tool` executes a normal tool call and resolves to the raw tool output object. Use it for shell and non-shell tools alike. Nested tool outputs stay inside JavaScript unless you emit them explicitly.\n");
     section.push_str("- `codex.emitImage(...)` adds one image to the outer `js_repl` function output each time you call it, so you can call it multiple times to emit multiple images. It accepts a data URL, a single `input_image` item, an object like `{ bytes, mimeType }`, or a raw tool response object with exactly one image and no text. It rejects mixed text-and-image content.\n");
+    section.push_str("- `codex.tool(...)` and `codex.emitImage(...)` keep stable helper identities across cells. Saved references and persisted objects can reuse them in later cells, but async callbacks that fire after a cell finishes still fail because no exec is active.\n");
     section.push_str("- Request full-resolution image processing with `detail: \"original\"` only when the `view_image` tool schema includes a `detail` argument. The same availability applies to `codex.emitImage(...)`: if `view_image.detail` is present, you may also pass `detail: \"original\"` there. Use this when high-fidelity image perception or precise localization is needed, especially for CUA agents.\n");
     section.push_str("- Example of sharing an in-memory Playwright screenshot: `await codex.emitImage({ bytes: await page.screenshot({ type: \"jpeg\", quality: 85 }), mimeType: \"image/jpeg\", detail: \"original\" })`.\n");
     section.push_str("- Example of sharing a local image tool result: `await codex.emitImage(codex.tool(\"view_image\", { path: \"/absolute/path\", detail: \"original\" }))`.\n");