Add saved image path hint to standalone image generation (#25947)

## 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`

codex-rs/core/src/stream_events_utils.rs

@@ -38,7 +38,8 @@ use tracing::warn;
 
 const GENERATED_IMAGE_ARTIFACTS_DIR: &str = "generated_images";
 
-pub(crate) fn image_generation_artifact_path(
+/// Returns the host-owned default artifact path for a generated image.
+pub fn image_generation_artifact_path(
     codex_home: &AbsolutePathBuf,
     session_id: &str,
     call_id: &str,