[codex] extend code-mode host IPC transport (#30108)

## Summary

- add an `EncodedFrame` type so IPC payloads are serialized and
size-checked before entering bounded queues
- add the V1 `operation/cancel` client-to-host message
- pin the new wire shape with protocol tests

## Why

The process-owned code-mode host needs bounded, pre-encoded outbound
messages and a best-effort cancellation signal. Keeping these wire
primitives in a protocol-only change lets their compatibility contract
be reviewed independently from either endpoint.

## Stack

This is **1 of 4** in the process-owned code-mode session stack. The
next PR targets this branch.

## Validation

- `just test -p codex-code-mode-protocol` — 22 passed
- `just fix -p codex-code-mode-protocol`
- `just fmt`

codex-rs/code-mode-protocol/src/host/codec.rs

@@ -11,6 +11,36 @@ use tokio::io::AsyncWriteExt;
 /// Maximum JSON payload size accepted for one IPC frame.
 pub const MAX_FRAME_BYTES: usize = 64 * 1024 * 1024;
 
+/// A serialized IPC frame that has already passed the payload size limit.
+#[derive(Clone, Debug)]
+pub struct EncodedFrame {
+    payload: Vec<u8>,
+}
+
+impl EncodedFrame {
+    pub fn encode<T>(message: &T) -> io::Result<Self>
+    where
+        T: Serialize,
+    {
+        let payload = serde_json::to_vec(message).map_err(|err| {
+            io::Error::new(
+                io::ErrorKind::InvalidData,
+                format!("failed to encode code-mode IPC frame: {err}"),
+            )
+        })?;
+        if payload.len() > MAX_FRAME_BYTES {
+            return Err(io::Error::new(
+                io::ErrorKind::InvalidData,
+                format!(
+                    "code-mode IPC frame length {} exceeds {MAX_FRAME_BYTES} bytes",
+                    payload.len()
+                ),
+            ));
+        }
+        Ok(Self { payload })
+    }
+}
+
 /// Decodes JSON messages prefixed by a four-byte little-endian payload length.
 pub struct FramedReader<R> {
     reader: R,
@@ -72,22 +102,12 @@ where
     where
         T: Serialize,
     {
-        let payload = serde_json::to_vec(message).map_err(|err| {
-            io::Error::new(
-                io::ErrorKind::InvalidData,
-                format!("failed to encode code-mode IPC frame: {err}"),
-            )
-        })?;
-        if payload.len() > MAX_FRAME_BYTES {
-            return Err(io::Error::new(
-                io::ErrorKind::InvalidData,
-                format!(
-                    "code-mode IPC frame length {} exceeds {MAX_FRAME_BYTES} bytes",
-                    payload.len()
-                ),
-            ));
-        }
-        let length = u32::try_from(payload.len()).map_err(|_| {
+        self.write_frame(&EncodedFrame::encode(message)?).await
+    }
+
+    /// Writes and flushes a frame encoded before it entered an I/O queue.
+    pub async fn write_frame(&mut self, frame: &EncodedFrame) -> io::Result<()> {
+        let length = u32::try_from(frame.payload.len()).map_err(|_| {
             io::Error::new(
                 io::ErrorKind::InvalidData,
                 "code-mode IPC frame length exceeds u32",
@@ -95,7 +115,7 @@ where
         })?;
 
         self.writer.write_all(&length.to_le_bytes()).await?;
-        self.writer.write_all(&payload).await?;
+        self.writer.write_all(&frame.payload).await?;
         self.writer.flush().await
     }
 }

codex-rs/code-mode-protocol/src/host/host_tests.rs

@@ -365,6 +365,16 @@ fn client_to_host_v1_variants_are_pinned() {
             }),
         );
     }
+
+    assert_wire_round_trip(
+        ClientToHost::CancelRequest {
+            id: request_id(/*value*/ 9),
+        },
+        json!({
+            "type": "operation/cancel",
+            "id": 9,
+        }),
+    );
 }
 
 #[test]

codex-rs/code-mode-protocol/src/host/message.rs

@@ -132,6 +132,8 @@ pub enum ClientToHost {
     ClientHello(ClientHello),
     #[serde(rename = "operation/request")]
     Request { id: RequestId, request: HostRequest },
+    #[serde(rename = "operation/cancel")]
+    CancelRequest { id: RequestId },
     #[serde(rename = "delegate/response")]
     DelegateResponse {
         id: DelegateRequestId,

codex-rs/code-mode-protocol/src/host/mod.rs

@@ -11,6 +11,7 @@ mod message;
 mod payload;
 mod types;
 
+pub use codec::EncodedFrame;
 pub use codec::FramedReader;
 pub use codec::FramedWriter;
 pub use codec::MAX_FRAME_BYTES;