[codex] replace remote images with model-visible error text (#29417)

## What

This PR will extend the existing centralized image-preparation path to
replace HTTP(S) image inputs with a model visible error message. It
won't "ruin" and break existing rollouts, but it will deprecate support
for the pathway. App server clients should no longer use HTTP image urls
if they'd like to upgrade.

The HTTP image url pathway is currently resolved in the responsesapi. It
is slow and not reccomended.

## Behavior

- HTTP(S) image URL: replace with `input_text`
- data URL: use the existing decode and resize path
- other image URL schemes: leave unchanged

This intentionally does not change app-server ingress. That validation
remains a follow-up.

## Test plan

- `just test -p codex-core -E
'test(/image_preparation|prepares_image_failures_before_history_insertion|prepares_resumed_history_before_installing_it|responses_lite_prepares_images/)'`
— 7 passed
- `just fix -p codex-core`
- `just fmt`

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

@@ -256,7 +256,7 @@ async fn standalone_image_edit_uses_attached_model_visible_image() -> Result<()>
 
 #[tokio::test]
 async fn standalone_image_edit_uses_recent_pathless_image() -> Result<()> {
-    let image_url = "https://example.com/reference.png";
+    let image_url = TINY_PNG_DATA_URL;
     let edit_request = run_image_edit_test(|_| {
         Ok((
             json!({