Run code mode exclusively through the standalone host (#36217)

## What changed

- Move the V8 implementation into a dedicated `codex-code-mode-runtime` crate used by `codex-code-mode-host`, removing the embedded runtime fallback from the Codex process.
- Resolve the host executable from the active installation layout and check its availability before selecting tools.
- Fall back to direct tools with a one-time warning when optional code mode is unavailable. Keep `code_mode_only` and `disable_in_process_fallback` configurations fail-closed.

## Testing

- Cover host discovery for standalone and package layouts, including missing hosts and symlinks.
- Verify direct-tool fallback, one-time warnings, and fail-closed code-mode-only behavior.

GitOrigin-RevId: 5aa3c6f1db148b2231fc24089a2ee0e2b00dbddb

codex-rs/core/src/session/review.rs

@@ -112,6 +112,7 @@ pub(super) async fn spawn_review_thread(
         sub_id: review_turn_id.clone(),
         trace_id: current_span_trace_id(),
         realtime_active: parent_turn_context.realtime_active,
+        code_mode_available: parent_turn_context.code_mode_available,
         config: per_turn_config,
         auth_manager: auth_manager_for_context,
         model_info: model_info.clone(),