code-mode standalone: extract protocol and add host crate (#27724)

This is phase 1 of a 4 phase stack:
1. **Add protocol and host crates for new IPC code mode implementation**
2. Create the new standalone binary
3. Create a new IPC `CodeModeSessionProvider` to use new binary
4. Remove v8 from core and only use IPC provider


## Add protocol and host crates for new IPC code mode implementation
Establish a clean process boundary without changing the existing
in-process behavior.

- Add the codex-code-mode-protocol crate for shared session, runtime,
response, and tool-definition types.
- Move protocol-facing code out of the V8-backed implementation.
- Add a buildable codex-code-mode-host crate as the foundation for the
standalone process.
- Keep the existing in-process runtime as the active implementation.

codex-rs/code-mode/src/service.rs

@@ -1,15 +1,29 @@
 use std::collections::HashMap;
-use std::fmt;
-use std::future::Future;
-use std::pin::Pin;
 use std::sync::Arc;
 use std::sync::atomic::AtomicBool;
 use std::sync::atomic::AtomicU64;
 use std::sync::atomic::Ordering;
 use std::time::Duration;
 
-use serde::Deserialize;
-use serde::Serialize;
+use codex_code_mode_protocol::CellId;
+use codex_code_mode_protocol::CodeModeNestedToolCall;
+use codex_code_mode_protocol::CodeModeSession;
+use codex_code_mode_protocol::CodeModeSessionDelegate;
+use codex_code_mode_protocol::CodeModeSessionProvider;
+use codex_code_mode_protocol::CodeModeSessionProviderFuture;
+use codex_code_mode_protocol::CodeModeSessionResultFuture;
+use codex_code_mode_protocol::DEFAULT_EXEC_YIELD_TIME_MS;
+use codex_code_mode_protocol::ExecuteRequest;
+use codex_code_mode_protocol::ExecuteToPendingOutcome;
+use codex_code_mode_protocol::FunctionCallOutputContentItem;
+use codex_code_mode_protocol::NotificationFuture;
+use codex_code_mode_protocol::RuntimeResponse;
+use codex_code_mode_protocol::StartedCell;
+use codex_code_mode_protocol::ToolInvocationFuture;
+use codex_code_mode_protocol::WaitOutcome;
+use codex_code_mode_protocol::WaitRequest;
+use codex_code_mode_protocol::WaitToPendingOutcome;
+use codex_code_mode_protocol::WaitToPendingRequest;
 use serde_json::Value as JsonValue;
 use tokio::sync::Mutex;
 use tokio::sync::mpsc;
@@ -18,88 +32,12 @@ use tokio::task::JoinSet;
 use tokio_util::sync::CancellationToken;
 use tracing::warn;
 
-use crate::FunctionCallOutputContentItem;
-use crate::runtime::CodeModeNestedToolCall;
-use crate::runtime::DEFAULT_EXEC_YIELD_TIME_MS;
-use crate::runtime::ExecuteRequest;
-use crate::runtime::ExecuteToPendingOutcome;
 use crate::runtime::PendingRuntimeMode;
 use crate::runtime::RuntimeCommand;
 use crate::runtime::RuntimeControlCommand;
 use crate::runtime::RuntimeEvent;
-use crate::runtime::RuntimeResponse;
-use crate::runtime::WaitOutcome;
-use crate::runtime::WaitRequest;
-use crate::runtime::WaitToPendingOutcome;
-use crate::runtime::WaitToPendingRequest;
 use crate::runtime::spawn_runtime;
 
-pub type CodeModeSessionResultFuture<'a, T> =
-    Pin<Box<dyn Future<Output = Result<T, String>> + Send + 'a>>;
-pub type CodeModeSessionProviderFuture<'a> =
-    CodeModeSessionResultFuture<'a, Arc<dyn CodeModeSession>>;
-pub type ToolInvocationFuture<'a> =
-    Pin<Box<dyn Future<Output = Result<JsonValue, String>> + Send + 'a>>;
-pub type NotificationFuture<'a> = Pin<Box<dyn Future<Output = Result<(), String>> + Send + 'a>>;
-
-#[derive(Clone, Debug, Deserialize, Eq, Hash, PartialEq, Serialize)]
-pub struct CellId(String);
-
-impl CellId {
-    pub fn new(value: String) -> Self {
-        Self(value)
-    }
-
-    pub fn as_str(&self) -> &str {
-        &self.0
-    }
-}
-
-impl AsRef<str> for CellId {
-    fn as_ref(&self) -> &str {
-        self.as_str()
-    }
-}
-
-impl fmt::Display for CellId {
-    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
-        formatter.write_str(self.as_str())
-    }
-}
-
-pub struct StartedCell {
-    pub cell_id: CellId,
-    initial_response_rx: oneshot::Receiver<RuntimeResponse>,
-}
-
-impl StartedCell {
-    pub async fn initial_response(self) -> Result<RuntimeResponse, String> {
-        self.initial_response_rx
-            .await
-            .map_err(|_| "exec runtime ended unexpectedly".to_string())
-    }
-}
-
-/// Host callbacks used by a code-mode session while cells are executing.
-pub trait CodeModeSessionDelegate: Send + Sync {
-    fn invoke_tool<'a>(
-        &'a self,
-        invocation: CodeModeNestedToolCall,
-        cancellation_token: CancellationToken,
-    ) -> ToolInvocationFuture<'a>;
-
-    fn notify<'a>(
-        &'a self,
-        call_id: String,
-        cell_id: CellId,
-        text: String,
-        cancellation_token: CancellationToken,
-    ) -> NotificationFuture<'a>;
-
-    /// Releases delegate state associated with a cell after it reaches a terminal state.
-    fn cell_closed(&self, cell_id: &CellId);
-}
-
 pub struct NoopCodeModeSessionDelegate;
 
 impl CodeModeSessionDelegate for NoopCodeModeSessionDelegate {
@@ -127,35 +65,6 @@ impl CodeModeSessionDelegate for NoopCodeModeSessionDelegate {
     fn cell_closed(&self, _cell_id: &CellId) {}
 }
 
-/// A durable code-mode session owned by one Codex thread.
-///
-/// Cells executed in the same session share stored values. Separate sessions
-/// must keep those values isolated. Implementations may execute cells
-/// in-process or remotely.
-pub trait CodeModeSession: Send + Sync {
-    fn execute<'a>(
-        &'a self,
-        request: ExecuteRequest,
-    ) -> CodeModeSessionResultFuture<'a, StartedCell>;
-
-    fn wait<'a>(&'a self, request: WaitRequest) -> CodeModeSessionResultFuture<'a, WaitOutcome>;
-
-    fn terminate<'a>(&'a self, cell_id: CellId) -> CodeModeSessionResultFuture<'a, WaitOutcome>;
-
-    fn shutdown<'a>(&'a self) -> CodeModeSessionResultFuture<'a, ()>;
-}
-
-/// Creates code-mode sessions for one Codex thread.
-///
-/// Providers choose where a session executes and receive the host delegate that
-/// the session should use for nested tool calls and notifications.
-pub trait CodeModeSessionProvider: Send + Sync {
-    fn create_session<'a>(
-        &'a self,
-        delegate: Arc<dyn CodeModeSessionDelegate>,
-    ) -> CodeModeSessionProviderFuture<'a>;
-}
-
 #[derive(Default)]
 pub struct InProcessCodeModeSessionProvider;
 
@@ -233,10 +142,7 @@ impl CodeModeService {
         )
         .await?;
 
-        Ok(StartedCell {
-            cell_id,
-            initial_response_rx: response_rx,
-        })
+        Ok(StartedCell::new(cell_id, response_rx))
     }
 
     pub async fn execute_to_pending(
@@ -432,6 +338,10 @@ impl Drop for CodeModeService {
 }
 
 impl CodeModeSession for CodeModeService {
+    fn is_alive(&self) -> bool {
+        !self.inner.shutting_down.load(Ordering::Acquire)
+    }
+
     fn execute<'a>(
         &'a self,
         request: ExecuteRequest,
@@ -876,10 +786,10 @@ mod tests {
     use super::WaitToPendingRequest;
     use super::run_cell_control;
     use crate::CodeModeToolKind;
+    use crate::ExecuteRequest;
+    use crate::ExecuteToPendingOutcome;
     use crate::FunctionCallOutputContentItem;
     use crate::ToolDefinition;
-    use crate::runtime::ExecuteRequest;
-    use crate::runtime::ExecuteToPendingOutcome;
     use crate::runtime::RuntimeEvent;
     use crate::runtime::spawn_runtime;