Add optional animated GIF output to image generation (#59073)

## What changed

- Gate GIF output behind `features.image_generation_animation`, disabled by default.
- Accept `gif` options for generation and editing, with optional `frame_count` and `frame_duration_ms` controls. Use `gif: {}` for API defaults.
- Preserve animated output with `.gif` filenames and `image/gif` data URLs while providing a still preview to the model.
- Add guidance for simple animations and spritesheet-based GIF edits.

## Testing

Add integration coverage for GIF generation and editing, explicit frame controls and API defaults, saved animation bytes, and still-preview replay across follow-up turns.

GitOrigin-RevId: 08afa0316ae3675921652157836191a649f563cf

codex-rs/features/src/lib.rs

@@ -384,6 +384,8 @@ pub enum Feature {
     ExternalMigration,
     /// Enable extension-backed image generation.
     ImageGeneration,
+    /// Allow the image-generation tool to request a backend-assembled GIF.
+    ImageGenerationAnimation,
     /// Omit inline image and audio content from app-server item notifications.
     OmitAppServerNotificationMedia,
     /// Tell the model when a prompt image was resized and include its dimensions.
@@ -632,6 +634,11 @@ impl Features {
         self.enabled.contains(&f)
     }
 
+    /// Whether to advertise GIF output in the image-generation tool.
+    pub fn image_generation_animation_enabled(&self) -> bool {
+        self.enabled(Feature::ImageGenerationAnimation)
+    }
+
     /// Whether a routing tier is enabled, independently of model catalog support.
     pub fn service_tier_enabled(&self, service_tier: &str) -> bool {
         match service_tier {
@@ -1962,6 +1969,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::ImageGenerationAnimation,
+        key: "image_generation_animation",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::OmitAppServerNotificationMedia,
         key: "omit_app_server_notification_media",