Support model-owned token budget defaults (#35608)

## What changed

- Add token-budget settings to model catalog messages and apply them when the
  feature is enabled without explicit token-budget configuration.
- Keep explicit user settings authoritative, reject invalid catalog defaults,
  and preserve resolved defaults in exported config locks.
- Manage context-window guidance through world state so it updates once when
  the active model changes while retaining prior conversation history.

## Testing

- Cover catalog defaults, explicit overrides, disabled features, invalid
  values, config-lock replay, and model switching.

GitOrigin-RevId: 54544fefaa14b09f5ef5bad9967a13c52b87b0fe

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

@@ -602,7 +602,7 @@ impl Session {
             )
         };
 
-        let config = Arc::new(config);
+        let mut config = Arc::new(config);
         let refresh_strategy = if session_source.is_non_root_agent() {
             codex_models_manager::manager::RefreshStrategy::Offline
         } else {
@@ -645,6 +645,11 @@ impl Session {
         let model_info = models_manager
             .get_model_info(model.as_str(), &config.to_models_manager_config())
             .await;
+        if config.config_lock_export_dir.is_some()
+            && config.config_lock_save_fields_resolved_from_model_catalog
+        {
+            self::token_budget::apply_model_defaults(Arc::make_mut(&mut config), &model_info);
+        }
         let multi_agent_version = config.multi_agent_version_override().or_else(|| {
             resolve_multi_agent_version(&conversation_history, inherited_multi_agent_version)
         });
@@ -3489,16 +3494,6 @@ impl Session {
                 )
                 .render(),
             );
-            if let Some(guidance_message) = turn_context
-                .config
-                .token_budget
-                .as_ref()
-                .and_then(|config| config.guidance_message.as_deref())
-                .filter(|message| !message.trim().is_empty())
-            {
-                developer_sections
-                    .push(crate::context::ContextWindowGuidance::new(guidance_message).render());
-            }
         }
         // Render the active mode after the usage hint so it can override that hint.
         let mut initial_multi_agent_mode = None;