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/Cargo.lock

@@ -3052,6 +3052,7 @@ dependencies = [
  "codex-model-provider-info",
  "codex-protocol",
  "codex-tools",
+ "codex-utils-absolute-path",
  "http 1.4.0",
  "pretty_assertions",
  "schemars 0.8.22",