fix: recover WeChat page thumbnail delivery

mindspace-image-generation.test.mjs

@@ -214,6 +214,55 @@ test('MindSpace image generation retries a semantic mismatch and stores only a r
   assert.match(generatedRequests[1].prompt, /缺失元素：竹海/);
 });
 
+test('WeChat page thumbnail idempotency follows the prompt while other keys stay stable', async () => {
+  const generatedRequests = [];
+  let assetCount = 0;
+  const service = createMindSpaceImageGenerationService({
+    configService: { async resolveImageGenerationPurpose() { return { ok: true, presetId: 'hero' }; } },
+    assetService: {
+      async createChatAsset() {
+        assetCount += 1;
+        return {
+          id: `asset-${assetCount}`,
+          workspaceRelativePath: `public/images/hero-${assetCount}.webp`,
+        };
+      },
+    },
+    imageMakeClient: {
+      async generateImage(input) {
+        generatedRequests.push(input);
+        return {
+          jobId: `job-${generatedRequests.length}`,
+          buffer: Buffer.from(`image-${generatedRequests.length}`),
+          mimeType: 'image/webp', sha256: 'abc', width: 768, height: 432,
+        };
+      },
+      async acknowledge() {},
+    },
+    imageReviewService: passingReviewService(),
+  });
+
+  const input = {
+    userId: 'user-1',
+    purpose: 'hero',
+    prompt: 'TKMind 舌战群儒，突出安全与落地',
+    idempotencyKey: 'wechat-25550597627048917-page-1-thumbnail',
+  };
+  await service.generate(input);
+  await service.generate(input);
+  await service.generate({ ...input, prompt: 'TKMind 舌战群儒，突出场景与思维进化' });
+  await service.generate({ ...input, idempotencyKey: 'ordinary-request', prompt: '普通图片' });
+
+  assert.equal(generatedRequests[0].idempotencyKey, generatedRequests[1].idempotencyKey);
+  assert.notEqual(generatedRequests[1].idempotencyKey, generatedRequests[2].idempotencyKey);
+  assert.match(
+    generatedRequests[0].idempotencyKey,
+    /^wechat-25550597627048917-page-1-thumbnail-p-[a-f0-9]{12}$/,
+  );
+  assert.ok(generatedRequests[0].idempotencyKey.length <= 128);
+  assert.equal(generatedRequests[3].idempotencyKey, 'ordinary-request');
+});
+
 test('MindSpace image generation blocks storage after all semantic review attempts fail', async () => {
   let generateCount = 0;
   let storeCount = 0;