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/app-server-protocol/schema/json/v2/ThreadStartedNotification.json

@@ -2131,6 +2131,33 @@
           "type": "object"
         },
         {
+          "anyOf": [
+            {
+              "properties": {
+                "fileId": {
+                  "type": "string"
+                }
+              },
+              "required": [
+                "fileId"
+              ],
+              "title": "FileIdThreadItem",
+              "type": "object"
+            },
+            {
+              "properties": {
+                "result": {
+                  "type": "string"
+                }
+              },
+              "required": [
+                "result"
+              ],
+              "title": "ResultThreadItem",
+              "type": "object"
+            }
+          ],
+          "description": "Either inline generated-image data or its durable attachment-store identity.\n\n`File` must remain first so rollouts written during the migration, which contain both an empty `result` and a `fileId`, retain the file reference when deserialized.",
           "properties": {
             "failure": {
               "anyOf": [
@@ -2146,9 +2173,6 @@
             "id": {
               "type": "string"
             },
-            "result": {
-              "type": "string"
-            },
             "revisedPrompt": {
               "type": [
                 "string",
@@ -2185,7 +2209,6 @@
           },
           "required": [
             "id",
-            "result",
             "status",
             "type"
           ],