fix(codex): await image URL fetches before sending to upstream (closes #575)

Remote HTTP(S) image URLs are fetched and inlined as base64 data URIs
in a new prefetchImages() step run before super.execute(), so the body
sent to Codex contains resolved image bytes instead of URLs the backend
cannot access.

Scope is limited to the Codex executor — base executor and other
providers are untouched.

Co-authored-by: anuragg-saxenaa <anuragg.saxenaa@gmail.com>
Made-with: Cursor

open-sse/translator/helpers/imageHelper.js

@@ -0,0 +1,34 @@
+/**
+ * Fetch a remote image URL and return it as a base64 data URI.
+ * Used when upstream providers (Codex, etc.) require inline base64 images
+ * instead of remote URLs they cannot fetch.
+ * Returns null if fetch fails.
+ *
+ * @param {string} imageUrl - HTTP(S) URL of the image
+ * @param {object} options - { signal, timeoutMs }
+ * @returns {Promise<{url: string, mimeType: string}|null>}
+ */
+export async function fetchImageAsBase64(imageUrl, options = {}) {
+  const { signal, timeoutMs = 10000 } = options;
+  if (!imageUrl || (!imageUrl.startsWith("http://") && !imageUrl.startsWith("https://"))) {
+    return null;
+  }
+
+  const controller = new AbortController();
+  const timeout = signal ? null : setTimeout(() => controller.abort(), timeoutMs);
+  const fetchSignal = signal || controller.signal;
+
+  try {
+    const response = await fetch(imageUrl, { signal: fetchSignal });
+    if (!response.ok) return null;
+
+    const mimeType = response.headers.get("Content-Type") || "image/jpeg";
+    const arrayBuffer = await response.arrayBuffer();
+    const base64 = Buffer.from(arrayBuffer).toString("base64");
+    return { url: `data:${mimeType};base64,${base64}`, mimeType };
+  } catch {
+    return null;
+  } finally {
+    if (timeout) clearTimeout(timeout);
+  }
+}