[codex] reject remote images at app-server ingress (#29419)

## Stack

Stacked on #29417. Review and land that PR first.

## Summary

- reject HTTP(S) image URLs in the handlers for `turn/start` and
`turn/steer`
- validate `thread/inject_items` after its existing
JSON-to-`ResponseItem` conversion, so each item is deserialized once
- turn invalid dynamic-tool image responses into the existing
unsuccessful text fallback; the model receives the validation message as
the function output
- leave `thread/resume.history` compatible with legacy history; #29417
replaces remote images before model input
- continue accepting inline data URLs and `localImage` inputs
- keep this policy in app-server; this PR does not add a shared protocol
API or change core image preparation

## Test plan

- `just test -p codex-app-server -E
'test(/request_handlers_reject_remote_image_urls|dynamic_tool_remote_image_response_becomes_model_visible_error|dynamic_tool_call_round_trip_sends_content_items_to_model|turn_start_tracks_turn_event_analytics|standalone_image_edit_uses_recent_pathless_image/)'`
(5 passed)
- `just fix -p codex-app-server`
- `just fmt`

codex-rs/app-server/src/image_url.rs

@@ -0,0 +1,8 @@
+pub(crate) const REMOTE_IMAGE_URL_ERROR: &str =
+    "remote image URLs are not supported; use an inline data URL instead";
+
+pub(crate) fn is_remote_image_url(image_url: &str) -> bool {
+    image_url.split_once(':').is_some_and(|(scheme, _)| {
+        scheme.eq_ignore_ascii_case("http") || scheme.eq_ignore_ascii_case("https")
+    })
+}