fix(brand-questions): surface seed topic min length as actionable error

Reject AI seed topics shorter than 2 characters on both client and server with
a dedicated invalid_seed_topic error code, and render an inline hint listing
the offending terms so users know exactly what to fix.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/admin-web/src/lib/errors.ts

@@ -140,6 +140,8 @@ export function isHandledAuthError(error: unknown): boolean {
 }
 
 const TIMEOUT_MESSAGE_PATTERN = /^timeout of \d+ms exceeded$/i
+const VALIDATION_MIN_SEED_TOPIC_PATTERN =
+  /QuestionDistillRequest\.SeedTopic|seed_topic.*min|SeedTopic.*min/i
 
 function translateRawErrorMessage(raw: string): string | null {
   const mapped = errorMessageMap[raw]
@@ -158,6 +160,13 @@ export function formatError(error: unknown): string {
   }
 
   if (error instanceof ApiClientError) {
+    if (
+      error.message === 'invalid_seed_topic' ||
+      (error.message === 'invalid_params' &&
+        VALIDATION_MIN_SEED_TOPIC_PATTERN.test(error.detail ?? ''))
+    ) {
+      return '用户搜索词太短，请输入至少 2 个字的搜索词'
+    }
     const translated = translateRawErrorMessage(error.message)
     const message = translated ?? error.message.replaceAll('_', ' ')
     if (isAiPointsInsufficient(error)) {