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/context/mod.rs

@@ -44,6 +44,7 @@ pub(crate) use environment_context::EnvironmentContext;
 pub(crate) use guardian_followup_review_reminder::GuardianFollowupReviewReminder;
 pub(crate) use hook_additional_context::HookAdditionalContext;
 pub(crate) use image_generation_instructions::ImageGenerationInstructions;
+pub use image_generation_instructions::extension_image_generation_output_hint;
 pub use internal_model_context::InternalContextSource;
 pub use internal_model_context::InternalModelContextFragment;
 pub use internal_model_context::InvalidInternalContextSource;