## Why
Standalone image generation returns image bytes to the model, but the
model also needs the host artifact path to reference the generated file
in follow-up work.
## What changed
- Append the default saved-image path hint alongside the generated image
tool output.
- Reuse the existing core image-generation hint text.
- Pass the thread ID and Codex home directory needed to compute the
artifact path.
- Add app-server and extension coverage for the model-visible hint.
## Validation
- `just fmt`
- `just bazel-lock-check`
- `just test -p codex-app-server
standalone_image_generation_returns_saved_path_hint_to_model`
## Summary
Restore Windows coverage for standalone image generation in code mode.
The previous test executed a V8-backed code-mode cell on Windows CI,
where that runtime path is intentionally excluded because it is
unreliable. The test was then ignored entirely on Windows, removing
useful coverage.
This splits the test into two checks:
- All platforms verify that `image_gen__imagegen` is exposed to the
model when image generation is configured for code mode only.
- Non-Windows platforms continue to execute the full V8-backed flow and
verify that the nested image-generation call succeeds.
## Verification
- `just fmt`
- `git diff --check`
- `just test -p codex-app-server standalone_image_generation`
Result: 3 tests passed, plus the required bench smoke check.
## Why
Standalone image generation remained top-level-only in code-mode
sessions.
## What changed
- Change imagegen exposure from `DirectModelOnly` to `Direct`.
- Keep direct-mode access while enabling nested code-mode access.
- Add a focused regression test for the exposure contract.
## Validation
- `just test -p codex-image-generation-extension`
Won Park