[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/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,