Support model catalog Apps usage instructions (#61213)

## What changed

- Add the disabled-by-default `catalog_apps_instructions` feature to use the selected model's `model_messages.apps.usage_instructions`, subject to existing Apps availability and model gates.
- Preserve valid catalog text literally, allow an empty string to clear guidance, and fall back to bundled instructions for missing, whitespace-only, or oversized values. Overrides are limited to 800 UTF-8 bytes.
- Reconcile guidance with the latest retained Apps fragment, emitting replacement or removal notices when needed and avoiding repeated updates across resumes. Preserve compatibility with bundled guidance and legacy checkpoints.
- Recognize catalog Apps fragments as contextual developer messages in event mapping and rollout rollback.

## Testing

Add resolver, world-state, client, and scenario coverage for literal text and byte limits, feature and availability gates, model switches, clearing guidance, resumes, and legacy history recovery.

GitOrigin-RevId: 6240d407ada8f22fa101753ff3ddb338632ae283

codex-rs/features/src/lib.rs

@@ -278,6 +278,8 @@ pub enum Feature {
     SpawnCsv,
     /// Enable apps.
     Apps,
+    /// Use Apps usage instructions from the selected model's catalog entry.
+    CatalogAppsInstructions,
     /// Route first-party ChatGPT requests through PSP.
     Psp,
     /// Enable MCP apps.
@@ -1726,6 +1728,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::CatalogAppsInstructions,
+        key: "catalog_apps_instructions",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::Psp,
         key: "psp",