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/core/src/session/mod.rs

@@ -31,6 +31,7 @@ use crate::environment_selection::TurnEnvironmentSnapshot;
 use crate::exec_policy::BANNED_PREFIX_SUGGESTIONS;
 use crate::exec_policy::ExecPolicyManager;
 use crate::exec_policy::default_policy_path;
+use crate::image_preparation::ImageResizeNoticeMode;
 use crate::image_preparation::prepare_response_items as prepare_image_response_items;
 use crate::parse_turn_item;
 use crate::realtime_conversation::RealtimeConversationManager;
@@ -1432,8 +1433,9 @@ impl Session {
         // Keep the recorded rollout unchanged. Prepare its reconstructed history before
         // installing it, so legacy media is processed once for this resume or fork and
         // will be processed again if the rollout is reconstructed in a future session.
-        // This meets media preparation requirements without modifying persisted rollouts.
-        prepare_image_response_items(&mut history);
+        // Never backfill resize notices during replay; only newly recorded items may
+        // emit them, so the historical model prefix remains unchanged.
+        let _ = prepare_image_response_items(&mut history, ImageResizeNoticeMode::Disabled);
         prepare_audio_response_items(&mut history);
         {
             let mut state = self.state.lock().await;
@@ -2898,13 +2900,25 @@ impl Session {
         turn_context: &TurnContext,
         items: &'a [ResponseItem],
     ) -> (Cow<'a, [ResponseItem]>, Vec<ImagePreparationMetadata>) {
-        let mut items = Cow::Borrowed(items);
-        let image_preparations = prepare_image_response_items(items.to_mut());
-        prepare_audio_response_items(items.to_mut());
+        let mut items = items.to_vec();
+        let image_preparations = prepare_image_response_items(
+            &mut items,
+            if turn_context
+                .config
+                .features
+                .enabled(Feature::ImageResizeNotice)
+            {
+                ImageResizeNoticeMode::Enabled
+            } else {
+                ImageResizeNoticeMode::Disabled
+            },
+        );
+        prepare_audio_response_items(&mut items);
         // Most response items get their passthrough turn ID at the durable history boundary.
-        for item in items.to_mut() {
+        for item in &mut items {
             item.set_turn_id_if_missing(&turn_context.sub_id);
         }
+        let items = Cow::Owned(items);
         (
             Self::assign_missing_response_item_ids(items),
             image_preparations,