## Why
#27920 stopped persisting image-generation items unless their status was
`completed`, preventing failed standalone extension items with empty
results from being saved. Built-in image generation can instead emit a
terminal `response.output_item.done` containing a complete base64 PNG
while the item status remains `generating`. In that case, app-server
emits no `savedPath`, so Codex Apps can render the inline image but
cannot expose a file artifact.
## What changed
- Persist image-generation items whenever `result` contains image data.
Failed terminal items still have empty results and remain unpersisted.
- Update the existing built-in image-generation integration test to
cover a terminal `generating` item and verify both `saved_path` and the
written PNG bytes.
## Validation
- Confirmed with a raw built-in websocket trace: the image progressed
through `in_progress`, `generating`, and `partial_image`, then emitted
one `response.output_item.done` with `status: "generating"` and a
complete PNG result.
- `just test -p codex-core builtin_image_generation_call_persisted` is
currently blocked before test execution by a pre-existing compile error
in `thread-store/src/thread_metadata_sync.rs:171`.
Won ParkandGitHub
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