Allow models to enable token budgeting by default (#41803)

## What changed

- Add model metadata flags for enabling token budgeting and its history-notes extension.
- Apply those activation defaults once at thread startup when the user has not explicitly configured token budgeting, while respecting managed feature requirements.
- Continue resolving model-owned prompts for the active model so model switches can use their own token-budget guidance.

## Testing

- Update the token-budget integration test to verify that model metadata activates the feature without manual configuration.

GitOrigin-RevId: 73a32623eaeabb9243e240c43da7a5e7e93e9a19

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

@@ -668,6 +668,10 @@ impl Session {
         let model_info = models_manager
             .get_model_info(model.as_str(), &config.to_models_manager_config())
             .await;
+        // Intentionally resolve `enabled` and `use_history_notes_extension` only at
+        // thread startup. Both activation flags stay fixed for this thread runtime,
+        // even if the selected model changes later.
+        token_budget::apply_model_defaults(Arc::make_mut(&mut config), &model_info);
         let configured_config = Arc::clone(&config);
         let multi_agent_version = config.multi_agent_version_override().or_else(|| {
             resolve_multi_agent_version(&conversation_history, inherited_multi_agent_version)