test(gateway): add regression test untuk makeImageCacheKey collision

Verifies that two data URLs sharing the first 128 chars (same MIME prefix
+ identical base64 header — the real-world scenario that caused ALL images
to reuse the same cached vision analysis) produce DIFFERENT cache keys
under the fixed full-dataURL hashing, whereas the old 128-char-prefix
approach would collide. Also includes consistency + prefix tests.

services/discord-gateway/tests/imageCacheKey.test.ts

@@ -0,0 +1,51 @@
+// ═══════════════════════════════════════════════════════════════════════════
+// makeImageCacheKey — regression for hash collision bug
+// ═══════════════════════════════════════════════════════════════════════════
+// Bug (2026-08-12): makeImageCacheKey() only hashed the first 128 chars of the
+// data URL. Since all resized images share the same MIME prefix
+// ('data:image/png;base64,') + identical base64 header bytes, nearly every
+// image got the SAME hash → 'image:<same-hash>' → all images reused the
+// first cached vision analysis ("konten judi").
+//
+// Fix: hash the ENTIRE data URL. This test verifies the fix and prevents
+// regression.
+import { createHash } from "node:crypto";
+import { describe, expect, it } from "vitest";
+import { makeImageCacheKey } from "../src/modules/ai-moderation/textCacheStore.js";
+
+function oldBuggyHash(dataUrl: string): string {
+  const prefix = dataUrl.slice(0, 128);
+  return "image:" + createHash("sha256").update(prefix).digest("hex").slice(0, 16);
+}
+
+describe("makeImageCacheKey — collision prevention", () => {
+  it("produces different keys for images whose first 128 chars are identical", () => {
+    // Two data URLs that SHARE the first 128 chars (same MIME + identical
+    // base64 header) but differ after — this is the real-world scenario
+    // that caused the collision bug.
+    const sharedPrefix =
+      "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg==" +
+      "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"; // pad to >128 chars
+
+    const imgA = sharedPrefix + "UNIQUE_TO_A";
+    const imgB = sharedPrefix + "UNIQUE_TO_B";
+
+    // Under the OLD buggy scheme: same prefix → same hash → COLLISION
+    expect(oldBuggyHash(imgA)).toBe(oldBuggyHash(imgB));
+
+    // Under the FIXED scheme: full data URL hashed → different keys
+    const keyA = makeImageCacheKey(imgA);
+    const keyB = makeImageCacheKey(imgB);
+    expect(keyA).not.toBe(keyB);
+  });
+
+  it("produces same key for identical input", () => {
+    const dataUrl = "data:image/png;base64,samebase64dataheremari";
+    expect(makeImageCacheKey(dataUrl)).toBe(makeImageCacheKey(dataUrl));
+  });
+
+  it("prefix is always 'image:'", () => {
+    const key = makeImageCacheKey("data:image/png;base64,test");
+    expect(key.startsWith("image:")).toBe(true);
+  });
+});