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

sdk/python/src/openai_codex/generated/v2_all.py

@@ -5524,18 +5524,35 @@ class SleepThreadItem(BaseModel):
     type: Annotated[Literal["sleep"], Field(title="SleepThreadItemType")]
 
 
-class ImageGenerationThreadItem(BaseModel):
+class FileIdThreadItem(BaseModel):
     model_config = ConfigDict(
         populate_by_name=True,
     )
     failure: ImageGenerationFailure | None = None
     id: str
-    result: str
     revised_prompt: Annotated[str | None, Field(alias="revisedPrompt")] = None
     saved_path: Annotated[AbsolutePathBuf | None, Field(alias="savedPath")] = None
     status: str
     transparent_background: Annotated[bool | None, Field(alias="transparentBackground")] = None
     type: Annotated[Literal["imageGeneration"], Field(title="ImageGenerationThreadItemType")]
+    file_id: Annotated[str, Field(alias="fileId")]
+
+
+class ResultThreadItem(BaseModel):
+    model_config = ConfigDict(
+        populate_by_name=True,
+    )
+    failure: ImageGenerationFailure | None = None
+    id: str
+    revised_prompt: Annotated[str | None, Field(alias="revisedPrompt")] = None
+    saved_path: Annotated[AbsolutePathBuf | None, Field(alias="savedPath")] = None
+    status: str
+    transparent_background: Annotated[bool | None, Field(alias="transparentBackground")] = None
+    type: Annotated[Literal["imageGeneration"], Field(title="ImageGenerationThreadItemType")]
+    result: str
+
+
+ImageGenerationThreadItem = ResultThreadItem
 
 
 class EnteredReviewModeThreadItem(BaseModel):
@@ -11647,7 +11664,8 @@ class ThreadItem(
         | WebSearchThreadItem
         | ImageViewThreadItem
         | SleepThreadItem
-        | ImageGenerationThreadItem
+        | FileIdThreadItem
+        | ResultThreadItem
         | EnteredReviewModeThreadItem
         | ExitedReviewModeThreadItem
         | ContextCompactionThreadItem
@@ -11672,7 +11690,8 @@ class ThreadItem(
         | WebSearchThreadItem
         | ImageViewThreadItem
         | SleepThreadItem
-        | ImageGenerationThreadItem
+        | FileIdThreadItem
+        | ResultThreadItem
         | EnteredReviewModeThreadItem
         | ExitedReviewModeThreadItem
         | ContextCompactionThreadItem