Code mode on v8 (#15276)

Moves Code Mode to a new crate with no dependencies on codex. This
create encodes the code mode semantics that we want for lifetime,
mounting, tool calling.

The model-facing surface is mostly unchanged. `exec` still runs raw
JavaScript, `wait` still resumes or terminates a `cell_id`, nested tools
are still available through `tools.*`, and helpers like `text`, `image`,
`store`, `load`, `notify`, `yield_control`, and `exit` still exist.

The major change is underneath that surface:

- Old code mode was an external Node runtime.
- New code mode is an in-process V8 runtime embedded directly in Rust.
- Old code mode managed cells inside a long-lived Node runner process.
- New code mode manages cells in Rust, with one V8 runtime thread per
active `exec`.
- Old code mode used JSON protocol messages over child stdin/stdout plus
Node worker-thread messages.
- New code mode uses Rust channels and direct V8 callbacks/events.

This PR also fixes the two migration regressions that fell out of that
substrate change:

- `wait { terminate: true }` now waits for the V8 runtime to actually
stop before reporting termination.
- synchronous top-level `exit()` now succeeds again instead of surfacing
as a script error.

---

- `core/src/tools/code_mode/*` is now mostly an adapter layer for the
public `exec` / `wait` tools.
- `code-mode/src/service.rs` owns cell sessions and async control flow
in Rust.
- `code-mode/src/runtime/*.rs` owns the embedded V8 isolate and
JavaScript execution.
- each `exec` spawns a dedicated runtime thread plus a Rust
session-control task.
- helper globals are installed directly into the V8 context instead of
being injected through a source prelude.
- helper modules like `tools.js` and `@openai/code_mode` are synthesized
through V8 module resolution callbacks in Rust.

---

Also added a benchmark for showing the speed of init and use of a code
mode env:
```
$ cargo bench -p codex-code-mode --bench exec_overhead -- --samples 30 --warm-iterations 25 --tool-counts 0,32,128
Finished [`bench` profile [optimized]](https://doc.rust-lang.org/cargo/reference/profiles.html#default-profiles) target(s) in 0.18s
     Running benches/exec_overhead.rs (target/release/deps/exec_overhead-008c440d800545ae)
exec_overhead: samples=30, warm_iterations=25, tool_counts=[0, 32, 128]
scenario       tools samples    warmups      iters      mean/exec       p95/exec       rssΔ p50       rssΔ max
cold_exec          0      30          0          1         1.13ms         1.20ms        8.05MiB        8.06MiB
warm_exec          0      30          1         25       473.43us       512.49us      912.00KiB        1.33MiB
cold_exec         32      30          0          1         1.03ms         1.15ms        8.08MiB        8.11MiB
warm_exec         32      30          1         25       509.73us       545.76us      960.00KiB        1.30MiB
cold_exec        128      30          0          1         1.14ms         1.19ms        8.30MiB        8.34MiB
warm_exec        128      30          1         25       575.08us       591.03us      736.00KiB      864.00KiB
memory uses a fresh-process max RSS delta for each scenario
```

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/code-mode/src/runtime/callbacks.rs

@@ -0,0 +1,209 @@
+use crate::response::FunctionCallOutputContentItem;
+
+use super::EXIT_SENTINEL;
+use super::RuntimeEvent;
+use super::RuntimeState;
+use super::value::json_to_v8;
+use super::value::normalize_output_image;
+use super::value::serialize_output_text;
+use super::value::throw_type_error;
+use super::value::v8_value_to_json;
+
+pub(super) fn tool_callback(
+    scope: &mut v8::PinScope<'_, '_>,
+    args: v8::FunctionCallbackArguments,
+    mut retval: v8::ReturnValue<v8::Value>,
+) {
+    let tool_name = args.data().to_rust_string_lossy(scope);
+    let input = if args.length() == 0 {
+        Ok(None)
+    } else {
+        v8_value_to_json(scope, args.get(0))
+    };
+    let input = match input {
+        Ok(input) => input,
+        Err(error_text) => {
+            throw_type_error(scope, &error_text);
+            return;
+        }
+    };
+
+    let Some(resolver) = v8::PromiseResolver::new(scope) else {
+        throw_type_error(scope, "failed to create tool promise");
+        return;
+    };
+    let promise = resolver.get_promise(scope);
+
+    let resolver = v8::Global::new(scope, resolver);
+    let Some(state) = scope.get_slot_mut::<RuntimeState>() else {
+        throw_type_error(scope, "runtime state unavailable");
+        return;
+    };
+    let id = format!("tool-{}", state.next_tool_call_id);
+    state.next_tool_call_id = state.next_tool_call_id.saturating_add(1);
+    let event_tx = state.event_tx.clone();
+    state.pending_tool_calls.insert(id.clone(), resolver);
+    let _ = event_tx.send(RuntimeEvent::ToolCall {
+        id,
+        name: tool_name,
+        input,
+    });
+    retval.set(promise.into());
+}
+
+pub(super) fn text_callback(
+    scope: &mut v8::PinScope<'_, '_>,
+    args: v8::FunctionCallbackArguments,
+    mut retval: v8::ReturnValue<v8::Value>,
+) {
+    let value = if args.length() == 0 {
+        v8::undefined(scope).into()
+    } else {
+        args.get(0)
+    };
+    let text = match serialize_output_text(scope, value) {
+        Ok(text) => text,
+        Err(error_text) => {
+            throw_type_error(scope, &error_text);
+            return;
+        }
+    };
+    if let Some(state) = scope.get_slot::<RuntimeState>() {
+        let _ = state.event_tx.send(RuntimeEvent::ContentItem(
+            FunctionCallOutputContentItem::InputText { text },
+        ));
+    }
+    retval.set(v8::undefined(scope).into());
+}
+
+pub(super) fn image_callback(
+    scope: &mut v8::PinScope<'_, '_>,
+    args: v8::FunctionCallbackArguments,
+    mut retval: v8::ReturnValue<v8::Value>,
+) {
+    let value = if args.length() == 0 {
+        v8::undefined(scope).into()
+    } else {
+        args.get(0)
+    };
+    let image_item = match normalize_output_image(scope, value) {
+        Ok(image_item) => image_item,
+        Err(()) => return,
+    };
+    if let Some(state) = scope.get_slot::<RuntimeState>() {
+        let _ = state.event_tx.send(RuntimeEvent::ContentItem(image_item));
+    }
+    retval.set(v8::undefined(scope).into());
+}
+
+pub(super) fn store_callback(
+    scope: &mut v8::PinScope<'_, '_>,
+    args: v8::FunctionCallbackArguments,
+    _retval: v8::ReturnValue<v8::Value>,
+) {
+    let key = match args.get(0).to_string(scope) {
+        Some(key) => key.to_rust_string_lossy(scope),
+        None => {
+            throw_type_error(scope, "store key must be a string");
+            return;
+        }
+    };
+    let value = args.get(1);
+    let serialized = match v8_value_to_json(scope, value) {
+        Ok(Some(value)) => value,
+        Ok(None) => {
+            throw_type_error(
+                scope,
+                &format!("Unable to store {key:?}. Only plain serializable objects can be stored."),
+            );
+            return;
+        }
+        Err(error_text) => {
+            throw_type_error(scope, &error_text);
+            return;
+        }
+    };
+    if let Some(state) = scope.get_slot_mut::<RuntimeState>() {
+        state.stored_values.insert(key, serialized);
+    }
+}
+
+pub(super) fn load_callback(
+    scope: &mut v8::PinScope<'_, '_>,
+    args: v8::FunctionCallbackArguments,
+    mut retval: v8::ReturnValue<v8::Value>,
+) {
+    let key = match args.get(0).to_string(scope) {
+        Some(key) => key.to_rust_string_lossy(scope),
+        None => {
+            throw_type_error(scope, "load key must be a string");
+            return;
+        }
+    };
+    let value = scope
+        .get_slot::<RuntimeState>()
+        .and_then(|state| state.stored_values.get(&key))
+        .cloned();
+    let Some(value) = value else {
+        retval.set(v8::undefined(scope).into());
+        return;
+    };
+    let Some(value) = json_to_v8(scope, &value) else {
+        throw_type_error(scope, "failed to load stored value");
+        return;
+    };
+    retval.set(value);
+}
+
+pub(super) fn notify_callback(
+    scope: &mut v8::PinScope<'_, '_>,
+    args: v8::FunctionCallbackArguments,
+    mut retval: v8::ReturnValue<v8::Value>,
+) {
+    let value = if args.length() == 0 {
+        v8::undefined(scope).into()
+    } else {
+        args.get(0)
+    };
+    let text = match serialize_output_text(scope, value) {
+        Ok(text) => text,
+        Err(error_text) => {
+            throw_type_error(scope, &error_text);
+            return;
+        }
+    };
+    if text.trim().is_empty() {
+        throw_type_error(scope, "notify expects non-empty text");
+        return;
+    }
+    if let Some(state) = scope.get_slot::<RuntimeState>() {
+        let _ = state.event_tx.send(RuntimeEvent::Notify {
+            call_id: state.tool_call_id.clone(),
+            text,
+        });
+    }
+    retval.set(v8::undefined(scope).into());
+}
+
+pub(super) fn yield_control_callback(
+    scope: &mut v8::PinScope<'_, '_>,
+    _args: v8::FunctionCallbackArguments,
+    _retval: v8::ReturnValue<v8::Value>,
+) {
+    if let Some(state) = scope.get_slot::<RuntimeState>() {
+        let _ = state.event_tx.send(RuntimeEvent::YieldRequested);
+    }
+}
+
+pub(super) fn exit_callback(
+    scope: &mut v8::PinScope<'_, '_>,
+    _args: v8::FunctionCallbackArguments,
+    _retval: v8::ReturnValue<v8::Value>,
+) {
+    if let Some(state) = scope.get_slot_mut::<RuntimeState>() {
+        state.exit_requested = true;
+    }
+    if let Some(error) = v8::String::new(scope, EXIT_SENTINEL) {
+        scope.throw_exception(error.into());
+    }
+}