Source async user message descriptions from the model catalog (#41461)

## What changed

- Add model message metadata for built-in tool descriptions.
- Use the active step model's catalog description for `send_user_message_async`, including after a mid-turn model change.
- Fall back to the built-in description when catalog metadata is absent, while preserving an explicitly empty description.

## Testing

- Cover catalog serialization, fallback and empty-value behavior, and model changes within a turn.

GitOrigin-RevId: 72b953214ab9708931065321debf12133f8d2d40

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

@@ -3467,13 +3467,14 @@ impl Session {
         });
         extension_data.insert(selected_plugins.clone());
         turn_context.extension_data.insert(selected_plugins);
-        // Tool planning still uses the admitted turn. Migrating it to the
-        // captured model is a separate step from diagnostic activation.
+        // Tool availability still follows the admitted turn; the async message
+        // description comes from the captured step model.
         let tool_router = turn::built_tools(
             self.as_ref(),
             turn_context.as_ref(),
             // TODO(CDXENT-441): use the step scoped model
             turn_context.model_info(),
+            settings.model_info.model_messages.as_ref(),
             &environments,
             &mcp,
             &extension_data,