[codex] Persist built-in image results reported as generating

[won-openai]

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.