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/tools/mod.rs

@@ -63,7 +63,7 @@ pub(crate) fn tool_user_shell_type(
     }
 }
 
-fn effective_tool_mode(turn_context: &TurnContext) -> ToolMode {
+pub(crate) fn requested_tool_mode(turn_context: &TurnContext) -> ToolMode {
     turn_context.model_info.tool_mode.unwrap_or_else(|| {
         if turn_context.config.features.enabled(Feature::CodeModeOnly) {
             ToolMode::CodeModeOnly
@@ -75,6 +75,18 @@ fn effective_tool_mode(turn_context: &TurnContext) -> ToolMode {
     })
 }
 
+pub(crate) fn effective_tool_mode(turn_context: &TurnContext) -> ToolMode {
+    let requested_tool_mode = requested_tool_mode(turn_context);
+    if !turn_context.code_mode_available
+        && requested_tool_mode == ToolMode::CodeMode
+        && !turn_context.config.code_mode.disable_in_process_fallback
+    {
+        ToolMode::Direct
+    } else {
+        requested_tool_mode
+    }
+}
+
 /// Format the combined exec output for sending back to the model.
 /// Includes exit code and duration metadata; truncates large bodies safely.
 pub fn format_exec_output_for_model(