Store generated image items as attachment references (#54398)

## What changed

- Upload generated images to the attachment store and expose `fileId` in image-generation items when available, with inline `result` fallback on upload or decoding failure.
- Preserve file references through legacy events, persisted rollouts, thread history, and task output summaries. Keep model-facing image preparation separate.
- Add attachment request tags and mark generated-image uploads with `usage=image_gen`, preserving the thread's ephemeral setting.
- Update public schemas and SDK types to accept either image representation, retain the Python `ImageGenerationThreadItem` alias, and prefer `fileId` when reading records containing both fields.

## Testing

Add coverage for file-backed generation and rollout persistence, legacy history replay, inline and transitional serialization, task output visibility, and Python SDK alias compatibility.

GitOrigin-RevId: f7a2bb5707e2f277348a547a988bc33c8e96d891

codex-rs/core/src/image_preparation.rs

@@ -1,3 +1,5 @@
+use std::collections::HashMap;
+
 use crate::config::ManagedFeatures;
 use crate::context::ContextualUserFragment;
 use crate::context::ImageResizeNotice;
@@ -370,6 +372,7 @@ async fn prepare_image(
         thread_id: origin.thread_id.to_owned(),
         ephemeral: origin.ephemeral,
         file_name: None,
+        tags: HashMap::new(),
         data: prepared.encoded.bytes.to_vec(),
     };
     *image = match image_store.upload(request).await {

codex-rs/core/src/tools/handlers/extension_tools.rs

@@ -212,6 +212,8 @@ async fn to_extension_call<'a>(
             session: Arc::downgrade(&invocation.session),
             turn: Arc::downgrade(&invocation.turn),
         }),
+        attachment_store: Arc::clone(&invocation.session.services.image_store),
+        ephemeral: invocation.turn.config.ephemeral,
         environments,
         payload: invocation.payload.clone(),
     }
@@ -222,6 +224,7 @@ mod tests {
     use std::sync::Arc;
 
     use codex_extension_items::ExtensionItem;
+    use codex_extension_items::image_generation::GeneratedImageRef;
     use codex_extension_items::image_generation::ImageGenerationItem;
     use codex_extension_items::web_search::WebSearchItem;
     use codex_protocol::items::TurnItem;
@@ -567,7 +570,9 @@ mod tests {
             id: "call-image".to_string(),
             status: "in_progress".to_string(),
             revised_prompt: None,
-            result: String::new(),
+            image: GeneratedImageRef::Inline {
+                result: String::new(),
+            },
             transparent_background: None,
             failure: None,
             saved_path: None,
@@ -578,7 +583,9 @@ mod tests {
             id: "call-image".to_string(),
             status: "completed".to_string(),
             revised_prompt: Some("A tiny blue square".to_string()),
-            result: "cG5n".to_string(),
+            image: GeneratedImageRef::Inline {
+                result: "cG5n".to_string(),
+            },
             transparent_background: Some(true),
             failure: None,
             saved_path: Some(expected_path.clone()),
@@ -603,7 +610,9 @@ mod tests {
                     call_id: "call-image".to_string(),
                     status: "completed".to_string(),
                     revised_prompt: Some("A tiny blue square".to_string()),
-                    result: "cG5n".to_string(),
+                    image: GeneratedImageRef::Inline {
+                        result: "cG5n".to_string(),
+                    },
                     transparent_background: Some(true),
                     failure: None,
                     saved_path: Some(expected_path.clone()),