task-26: 模块 API 快照契约测试基建

新增集中式 assertApiCall() helper（tests/helpers/api-snapshot.ts）：
断言 url/method/params/data 快照，params/data 白名单外字段报错，
失败信息含实际与期望 URL 上下文；isApiModule() 覆盖 16 个已拆模块。

8 个测试：helper url/method/params/body、all_modules_covered、
fail_message、allowlist、regression_guard。

frontend-vue/tests/helpers/api-snapshot.ts

@@ -0,0 +1,58 @@
+import assert from 'node:assert/strict'
+
+export interface ApiCallSnapshot {
+  url?: string
+  method?: string
+  params?: Record<string, unknown>
+  data?: unknown
+}
+
+const ALLOWED_SECTIONS = ['url', 'method', 'params', 'data'] as const
+
+const apiModules = [
+  'dedupe', 'split', 'convert', 'publish', 'similar-asin', 'appearance-patent',
+  'delete-brand', 'price-track', 'shop-match', 'query-asin', 'withdraw',
+  'collect-data', 'image-video', 'brand', 'permission', 'digital-human',
+] as const
+
+export function isApiModule(name: string): boolean {
+  return (apiModules as readonly string[]).includes(name)
+}
+
+export function assertApiCall(calls: ApiCallSnapshot[], index: number, expected: ApiCallSnapshot) {
+  const actual = calls[index]
+  assert.ok(actual, `第 ${index} 次请求不存在（共 ${calls.length} 次）`)
+  const context = `第 ${index} 次请求实际=${JSON.stringify(actual)} 期望=${JSON.stringify(expected)}`
+  for (const section of ALLOWED_SECTIONS) {
+    assert.ok(
+      !(section in expected) || section in actual,
+      `快照缺少 section: ${section}。${context}`,
+    )
+  }
+  if (expected.url !== undefined) {
+    assert.equal(actual.url, expected.url, `URL 快照不匹配。${context}`)
+  }
+  if (expected.method !== undefined) {
+    assert.equal(actual.method, expected.method, `method 快照不匹配。${context}`)
+  }
+  if (expected.params !== undefined) {
+    const actualParams = actual.params ?? {}
+    for (const key of Object.keys(actualParams)) {
+      assert.ok(
+        key in expected.params,
+        `params 出现白名单外字段: ${key}。${context}`,
+      )
+    }
+    assert.deepEqual(actualParams, expected.params, `params 快照不匹配。${context}`)
+  }
+  if (expected.data !== undefined) {
+    const actualData = (actual.data ?? {}) as Record<string, unknown>
+    for (const key of Object.keys(actualData)) {
+      assert.ok(
+        key in expected.data,
+        `data 出现白名单外字段: ${key}。${context}`,
+      )
+    }
+    assert.deepEqual(actualData, expected.data, `data 快照不匹配。${context}`)
+  }
+}