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/image_generation_instructions.rs

@@ -1,6 +1,27 @@
 use super::ContextualUserFragment;
 use std::fmt::Display;
 
+/// Maximum size of the extension's model-facing generated-image path hint.
+const MAX_IMAGE_GENERATION_OUTPUT_HINT_BYTES: usize = 1024;
+
+/// Returns the extension's model-facing hint, or omits it if the path makes it too large.
+pub fn extension_image_generation_output_hint(
+    image_output_dir: impl Display,
+    image_output_path: impl Display,
+) -> Option<String> {
+    let hint = image_generation_hint(image_output_dir, image_output_path);
+    (hint.len() <= MAX_IMAGE_GENERATION_OUTPUT_HINT_BYTES).then_some(hint)
+}
+
+fn image_generation_hint(
+    image_output_dir: impl Display,
+    image_output_path: impl Display,
+) -> String {
+    format!(
+        "Generated images are saved to {image_output_dir} as {image_output_path} by default.\nIf you need to use a generated image at another path, copy it and leave the original in place unless the user explicitly asks you to delete it."
+    )
+}
+
 #[derive(Debug, Clone, PartialEq)]
 pub(crate) struct ImageGenerationInstructions {
     image_output_dir: String,
@@ -30,9 +51,6 @@ impl ContextualUserFragment for ImageGenerationInstructions {
     }
 
     fn body(&self) -> String {
-        format!(
-            "Generated images are saved to {} as {} by default.\nIf you need to use a generated image at another path, copy it and leave the original in place unless the user explicitly asks you to delete it.",
-            self.image_output_dir, self.image_output_path
-        )
+        image_generation_hint(&self.image_output_dir, &self.image_output_path)
     }
 }

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;

codex-rs/core/src/lib.rs

@@ -99,6 +99,7 @@ pub(crate) use skills::manager;
 pub(crate) use skills::maybe_emit_implicit_skill_invocation;
 pub(crate) use skills::skills_load_input_from_config;
 mod stream_events_utils;
+pub use stream_events_utils::image_generation_artifact_path;
 pub mod test_support;
 mod unified_exec;
 pub mod windows_sandbox;

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,