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-protocol/src/session_tests.rs

@@ -0,0 +1,19 @@
+use pretty_assertions::assert_eq;
+use tokio::sync::oneshot;
+
+use super::CellId;
+use super::StartedCell;
+
+#[tokio::test]
+async fn started_cell_preserves_remote_initial_response_errors() {
+    let (response_tx, response_rx) = oneshot::channel();
+    response_tx
+        .send(Err("remote runtime failed".to_string()))
+        .expect("initial response receiver should be open");
+    let started = StartedCell::from_result_receiver(CellId::new("1".to_string()), response_rx);
+
+    assert_eq!(
+        started.initial_response().await,
+        Err("remote runtime failed".to_string())
+    );
+}