Report prompt image resizing to the model (#37134)

## What changed

- Add the disabled-by-default `image_resize_notice` feature.
- When enabled, append a developer message after resized images from user
  messages or tool outputs. The notice identifies each image and reports its
  original and prepared dimensions.
- Persist notices with new history while avoiding backfilled notices for images
  processed when a session is resumed.

## Testing

- Cover notice placement, image numbering, failed images, user and tool image
  sources, and resumed-session replay.

GitOrigin-RevId: 11dd808b3ad47eac35e9e2f4a4370f85d0122770

codex-rs/features/src/lib.rs

@@ -222,6 +222,8 @@ pub enum Feature {
     ExternalMigration,
     /// Enable extension-backed image generation.
     ImageGeneration,
+    /// Tell the model when a prompt image was resized and include its dimensions.
+    ImageResizeNotice,
     /// Removed compatibility flag for always-on centralized image preparation.
     ResizeAllImages,
     /// Removed compatibility flag for always-on response item IDs.
@@ -1268,6 +1270,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::ImageResizeNotice,
+        key: "image_resize_notice",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::ResizeAllImages,
         key: "resize_all_images",