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/turn_context.rs

@@ -115,6 +115,7 @@ pub struct TurnContext {
     pub(crate) sub_id: String,
     pub(crate) trace_id: Option<String>,
     pub(crate) realtime_active: bool,
+    pub(crate) code_mode_available: bool,
     pub config: Arc<Config>,
     pub(crate) auth_manager: Option<Arc<AuthManager>>,
     pub(crate) model_info: ModelInfo,
@@ -280,6 +281,7 @@ impl TurnContext {
             sub_id: self.sub_id.clone(),
             trace_id: self.trace_id.clone(),
             realtime_active: self.realtime_active,
+            code_mode_available: self.code_mode_available,
             config: Arc::new(config),
             auth_manager: self.auth_manager.clone(),
             model_info: model_info.clone(),
@@ -551,6 +553,7 @@ impl Session {
             sub_id,
             trace_id: current_span_trace_id(),
             realtime_active: false,
+            code_mode_available: true,
             config: per_turn_config,
             auth_manager: auth_manager_for_context,
             model_info,
@@ -798,6 +801,7 @@ impl Session {
             sub_id,
             skills_snapshot,
         );
+        turn_context.code_mode_available = self.services.code_mode_service.is_available();
         turn_context.extension_data.insert(trusted_plugin_roots);
         turn_context.realtime_active = self.conversation.running_state().await.is_some();
 
@@ -830,6 +834,21 @@ impl Session {
             .await;
         }
 
+        if !tc.code_mode_available
+            && matches!(
+                crate::tools::requested_tool_mode(tc),
+                codex_protocol::openai_models::ToolMode::CodeMode
+                    | codex_protocol::openai_models::ToolMode::CodeModeOnly
+            )
+            && let Some(message) = self
+                .services
+                .code_mode_service
+                .take_unavailable_warning(crate::tools::effective_tool_mode(tc))
+        {
+            self.send_event(tc, EventMsg::Warning(WarningEvent { message }))
+                .await;
+        }
+
         if let Some(message) =
             unsupported_code_mode_warning(&tc.model_info, tc.config.features.get())
         {