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/runtime.rs

@@ -0,0 +1,89 @@
+use codex_protocol::ToolName;
+use serde::Deserialize;
+use serde::Serialize;
+use serde_json::Value as JsonValue;
+
+use crate::CellId;
+use crate::CodeModeToolKind;
+use crate::FunctionCallOutputContentItem;
+use crate::ToolDefinition;
+
+pub const DEFAULT_EXEC_YIELD_TIME_MS: u64 = 10_000;
+pub const DEFAULT_WAIT_YIELD_TIME_MS: u64 = 10_000;
+pub const DEFAULT_MAX_OUTPUT_TOKENS_PER_EXEC_CALL: usize = 10_000;
+
+#[derive(Clone, Debug, Deserialize, PartialEq, Serialize)]
+pub struct ExecuteRequest {
+    pub tool_call_id: String,
+    pub enabled_tools: Vec<ToolDefinition>,
+    pub source: String,
+    pub yield_time_ms: Option<u64>,
+    pub max_output_tokens: Option<usize>,
+}
+
+#[derive(Clone, Debug, Deserialize, PartialEq, Serialize)]
+pub struct WaitRequest {
+    pub cell_id: CellId,
+    pub yield_time_ms: u64,
+}
+
+#[derive(Clone, Debug, Deserialize, PartialEq, Serialize)]
+pub struct WaitToPendingRequest {
+    pub cell_id: CellId,
+}
+
+#[derive(Debug, Deserialize, PartialEq, Serialize)]
+pub enum WaitOutcome {
+    LiveCell(RuntimeResponse),
+    MissingCell(RuntimeResponse),
+}
+
+#[derive(Debug, Deserialize, PartialEq, Serialize)]
+pub enum ExecuteToPendingOutcome {
+    Pending {
+        cell_id: CellId,
+        content_items: Vec<FunctionCallOutputContentItem>,
+        pending_tool_call_ids: Vec<String>,
+    },
+    Completed(RuntimeResponse),
+}
+
+#[derive(Debug, Deserialize, PartialEq, Serialize)]
+pub enum WaitToPendingOutcome {
+    LiveCell(ExecuteToPendingOutcome),
+    MissingCell(RuntimeResponse),
+}
+
+impl From<WaitOutcome> for RuntimeResponse {
+    fn from(outcome: WaitOutcome) -> Self {
+        match outcome {
+            WaitOutcome::LiveCell(response) | WaitOutcome::MissingCell(response) => response,
+        }
+    }
+}
+
+#[derive(Clone, Debug, Deserialize, PartialEq, Serialize)]
+pub enum RuntimeResponse {
+    Yielded {
+        cell_id: CellId,
+        content_items: Vec<FunctionCallOutputContentItem>,
+    },
+    Terminated {
+        cell_id: CellId,
+        content_items: Vec<FunctionCallOutputContentItem>,
+    },
+    Result {
+        cell_id: CellId,
+        content_items: Vec<FunctionCallOutputContentItem>,
+        error_text: Option<String>,
+    },
+}
+
+#[derive(Clone, Debug, Deserialize, PartialEq, Serialize)]
+pub struct CodeModeNestedToolCall {
+    pub cell_id: CellId,
+    pub runtime_tool_call_id: String,
+    pub tool_name: ToolName,
+    pub tool_kind: CodeModeToolKind,
+    pub input: Option<JsonValue>,
+}