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/app-server/tests/suite/v2/imagegen_extension.rs

@@ -44,7 +44,7 @@ const DEFAULT_READ_TIMEOUT: Duration = Duration::from_secs(60);
 const DEFAULT_READ_TIMEOUT: Duration = Duration::from_secs(10);
 
 #[tokio::test]
-async fn standalone_image_generation_persists_image_and_returns_it_to_model() -> Result<()> {
+async fn standalone_image_generation_returns_saved_path_hint_to_model() -> Result<()> {
     let call_id = "image-run-1";
     let server = responses::start_mock_server().await;
     mount_image_response(&server).await;
@@ -124,7 +124,13 @@ async fn standalone_image_generation_persists_image_and_returns_it_to_model() ->
             "detail": "high",
         })
     );
-    assert_eq!(output["output"].as_array().map(Vec::len), Some(1));
+    let output_hint = output["output"][1]["text"]
+        .as_str()
+        .context("image output should include model-visible path hint")?;
+    assert!(
+        output_hint.contains(&saved_path.display().to_string()),
+        "output hint should identify the path core saved"
+    );
     assert!(
         !requests[1]
             .message_input_texts("developer")
@@ -199,7 +205,7 @@ const result = await tools.image_gen__imagegen({
   action: "generate",
   prompt: "paint a blue whale",
 });
-image(result);
+generatedImage(result);
 "#,
                 ),
                 responses::ev_completed("resp-1"),
@@ -246,7 +252,12 @@ image(result);
             "detail": "high",
         })
     );
-    assert_eq!(output["output"].as_array().map(Vec::len), Some(2));
+    assert!(
+        output["output"][2]["text"]
+            .as_str()
+            .is_some_and(|text| text.contains("Generated images are saved"))
+    );
+    assert_eq!(output["output"].as_array().map(Vec::len), Some(3));
 
     Ok(())
 }