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/response.rs

@@ -0,0 +1,26 @@
+use serde::Deserialize;
+use serde::Serialize;
+
+#[derive(Debug, Clone, Copy, Serialize, Deserialize, PartialEq, Eq)]
+#[serde(rename_all = "lowercase")]
+pub enum ImageDetail {
+    Auto,
+    Low,
+    High,
+    Original,
+}
+
+pub const DEFAULT_IMAGE_DETAIL: ImageDetail = ImageDetail::High;
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+#[serde(tag = "type", rename_all = "snake_case")]
+pub enum FunctionCallOutputContentItem {
+    InputText {
+        text: String,
+    },
+    InputImage {
+        image_url: String,
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        detail: Option<ImageDetail>,
+    },
+}