## Why
Standalone image generation emitted a started item but no terminal item
when the backend failed. Clients could leave the operation unresolved or
render it as successful.
## What changed
- Emit a terminal image-generation item with `status: "failed"` when
generation or editing fails.
- Skip image persistence for failed terminal items.
- Render failed image generation distinctly in TUI history.
- Preserve the status when handling live and replayed terminal items.
## Looks for TUI, App-Side change needed
<img width="867" height="89" alt="image"
src="https://github.com/user-attachments/assets/9e32342f-a982-411e-8498-456639fc468a"
/>
## Validation
- `just test -p codex-image-generation-extension`
- App-server image-generation tests
- Core stream-event tests
- TUI image-generation lifecycle and snapshot tests
- Scoped Clippy and formatting
Won ParkandGitHub
b6baa77eec