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/config_lock.rs

@@ -157,7 +157,9 @@ fn save_config_resolved_fields(
         resolved_config_to_toml(&config.multi_agent_v2, "features.multi_agent_v2")?;
     multi_agent_v2.enabled = Some(config.features.enabled(Feature::MultiAgentV2));
     features.multi_agent_v2 = Some(FeatureToml::Config(multi_agent_v2));
-    if let Some(token_budget) = config.token_budget.as_ref() {
+    if let Some(token_budget) = config.token_budget.as_ref()
+        && super::token_budget::has_explicit_settings(config)
+    {
         let mut token_budget: TokenBudgetConfigToml =
             resolved_config_to_toml(token_budget, "features.token_budget")?;
         token_budget.enabled = Some(config.features.enabled(Feature::TokenBudget));
@@ -387,6 +389,29 @@ mod tests {
         assert_eq!(lockfile.version, crate::config_lock::CONFIG_LOCK_VERSION);
     }
 
+    #[tokio::test]
+    async fn lock_preserves_model_owned_token_budget_defaults() {
+        let mut sc = crate::session::tests::make_session_configuration_for_tests().await;
+        let mut config = (*sc.original_config_do_not_use).clone();
+        config.token_budget = Some(crate::config::TokenBudgetConfig::default());
+        config
+            .features
+            .enable(Feature::TokenBudget)
+            .expect("token_budget should be enableable in tests");
+        sc.original_config_do_not_use = Arc::new(config);
+
+        let lockfile = sc.to_config_lockfile_toml().expect("lock should serialize");
+
+        assert_eq!(
+            lockfile
+                .config
+                .features
+                .as_ref()
+                .and_then(|features| features.token_budget.as_ref()),
+            Some(&FeatureToml::Enabled(true))
+        );
+    }
+
     #[tokio::test]
     async fn lock_skips_session_values_when_model_catalog_fields_are_not_saved() {
         let mut sc = crate::session::tests::make_session_configuration_for_tests().await;