Expose image generation usage-limit failures (#38024)

## What changed

- Add structured `usageLimitExceeded` metadata to failed image-generation
  items, including the image limit ID and optional reset timestamp.
- Preserve the failure metadata in completion events, app-server thread
  history, thread reads and resumes, and migrated legacy rollouts.
- Export the new failure type in the generated JSON and TypeScript schemas.

## Testing

- Cover usage-limit responses through item completion, persistence, and thread
  resume.
- Verify legacy rollout migration retains image-generation failure metadata.

GitOrigin-RevId: 91c31a2a47456b6b916ee03002ff314b05946dd4

codex-rs/ext/items/src/image_generation.rs

@@ -4,6 +4,21 @@ use serde::Deserialize;
 use serde::Serialize;
 use ts_rs::TS;
 
+#[derive(Debug, Clone, Deserialize, Serialize, TS, JsonSchema, PartialEq)]
+#[serde(tag = "type", rename_all = "camelCase")]
+#[ts(tag = "type", rename_all = "camelCase")]
+pub enum ImageGenerationFailure {
+    UsageLimitExceeded {
+        #[serde(rename = "limitId")]
+        #[ts(rename = "limitId")]
+        limit_id: String,
+        #[serde(rename = "resetsAt")]
+        #[ts(rename = "resetsAt")]
+        #[ts(type = "number | null")]
+        resets_at: Option<i64>,
+    },
+}
+
 // Standalone image-generation item owned by the image extension. This is also
 // the field-level representation exposed by app-server; core and rollout
 // persistence only carry it inside an ExtensionItem envelope.
@@ -18,6 +33,8 @@ pub struct ImageGenerationItem {
     #[serde(default)]
     #[ts(optional)]
     pub transparent_background: Option<bool>,
+    #[serde(default)]
+    pub failure: Option<ImageGenerationFailure>,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     #[ts(optional)]
     pub saved_path: Option<AbsolutePathBuf>,