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/models-manager/src/model_info.rs

@@ -115,6 +115,7 @@ fn clear_instruction_messages(model: &mut ModelInfo) {
         if model_messages.approvals.is_none()
             && model_messages.auto_review.is_none()
             && model_messages.permissions.is_none()
+            && model_messages.token_budget.is_none()
         {
             model.model_messages = None;
         }
@@ -181,6 +182,7 @@ fn local_personality_messages_for_slug(slug: &str) -> Option<ModelMessages> {
             approvals: None,
             auto_review: None,
             permissions: None,
+            token_budget: None,
         }),
         _ => None,
     }