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/lib.rs

@@ -0,0 +1,45 @@
+mod description;
+mod response;
+mod runtime;
+mod session;
+
+pub use description::CODE_MODE_PRAGMA_PREFIX;
+pub use description::CodeModeToolKind;
+pub use description::EnabledToolMetadata;
+pub use description::ToolDefinition;
+pub use description::ToolNamespaceDescription;
+pub use description::augment_tool_definition;
+pub use description::build_exec_tool_description;
+pub use description::build_wait_tool_description;
+pub use description::enabled_tool_metadata;
+pub use description::is_code_mode_nested_tool;
+pub use description::normalize_code_mode_identifier;
+pub use description::parse_exec_source;
+pub use description::render_code_mode_sample;
+pub use description::render_json_schema_to_typescript;
+pub use response::DEFAULT_IMAGE_DETAIL;
+pub use response::FunctionCallOutputContentItem;
+pub use response::ImageDetail;
+pub use runtime::CodeModeNestedToolCall;
+pub use runtime::DEFAULT_EXEC_YIELD_TIME_MS;
+pub use runtime::DEFAULT_MAX_OUTPUT_TOKENS_PER_EXEC_CALL;
+pub use runtime::DEFAULT_WAIT_YIELD_TIME_MS;
+pub use runtime::ExecuteRequest;
+pub use runtime::ExecuteToPendingOutcome;
+pub use runtime::RuntimeResponse;
+pub use runtime::WaitOutcome;
+pub use runtime::WaitRequest;
+pub use runtime::WaitToPendingOutcome;
+pub use runtime::WaitToPendingRequest;
+pub use session::CellId;
+pub use session::CodeModeSession;
+pub use session::CodeModeSessionDelegate;
+pub use session::CodeModeSessionProvider;
+pub use session::CodeModeSessionProviderFuture;
+pub use session::CodeModeSessionResultFuture;
+pub use session::NotificationFuture;
+pub use session::StartedCell;
+pub use session::ToolInvocationFuture;
+
+pub const PUBLIC_TOOL_NAME: &str = "exec";
+pub const WAIT_TOOL_NAME: &str = "wait";