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/views/BrandQuestionCreateView.vue

@@ -30,6 +30,8 @@ interface CandidateRow extends QuestionCandidate {
   selected: boolean
 }
 
+const AI_SEED_TOPIC_MIN_LENGTH = 2
+
 const queryClient = useQueryClient()
 const route = useRoute()
 const router = useRouter()
@@ -44,6 +46,7 @@ const aiCandidateModalOpen = ref(false)
 const aiCandidateProgress = ref(12)
 const aiCandidateStageIndex = ref(0)
 const aiWaitingKind = ref<'candidate' | 'fill'>('candidate')
+const aiSeedTopicError = ref('')
 
 let aiCandidateProgressTimer: number | null = null
 let aiCandidateCloseTimer: number | null = null
@@ -269,7 +272,7 @@ const previewMutations = {
       currentStep.value = 1
     },
     onError: (error) => {
-      pageNotice.value = formatError(error)
+      handleAiPreviewError(error)
     },
   }),
 }
@@ -366,7 +369,14 @@ const normalizedAiSeedTopics = computed(() => {
   return topics
 })
 
+const tooShortAiSeedTopics = computed(() =>
+  normalizedAiSeedTopics.value.filter(
+    (topic) => countSearchTermChars(topic) < AI_SEED_TOPIC_MIN_LENGTH,
+  ),
+)
+
 function updateAiSeedTopics(value: string[]): void {
+  clearAiSeedTopicError()
   aiForm.seed_topics.splice(0, aiForm.seed_topics.length, ...dedupeTopicParts(value))
 }
 
@@ -506,6 +516,10 @@ function normalizeSearchTermText(value: string): string {
     .replace(/\s+/g, '')
 }
 
+function countSearchTermChars(value: string): number {
+  return Array.from(normalizeSearchTermText(value)).length
+}
+
 function searchTermLooksValid(value: string): boolean {
   const text = normalizeSearchTermText(value)
   if (text.length < 4) {
@@ -565,12 +579,63 @@ function changeCreateMode(mode: CreateMode): void {
   if (createMode.value === mode) {
     return
   }
+  clearAiSeedTopicError()
   createMode.value = mode
   currentStep.value = 0
   candidateRows.value = []
   pageNotice.value = ''
 }
 
+function setAiSeedTopicError(value: string): void {
+  aiSeedTopicError.value = value
+  pageNotice.value = value
+}
+
+function clearAiSeedTopicError(): void {
+  if (aiSeedTopicError.value && pageNotice.value === aiSeedTopicError.value) {
+    pageNotice.value = ''
+  }
+  aiSeedTopicError.value = ''
+}
+
+function formatQuotedTerms(terms: string[]): string {
+  return terms
+    .slice(0, 3)
+    .map((term) => `“${term}”`)
+    .join('、')
+}
+
+function buildAiSeedTopicTooShortMessage(terms: string[]): string {
+  const visibleTerms = formatQuotedTerms(terms)
+  const hiddenCount = Math.max(terms.length - 3, 0)
+  if (hiddenCount > 0) {
+    return t('brands.questions.errors.aiSeedTopicTooShortWithMore', {
+      terms: visibleTerms,
+      count: hiddenCount,
+    })
+  }
+  return t('brands.questions.errors.aiSeedTopicTooShort', {
+    terms: visibleTerms,
+  })
+}
+
+function isAiSeedTopicValidationError(error: unknown): boolean {
+  return (
+    error instanceof ApiClientError &&
+    (error.message === 'invalid_seed_topic' ||
+      (error.message === 'invalid_params' && /SeedTopic|seed_topic/.test(error.detail ?? '')))
+  )
+}
+
+function handleAiPreviewError(error: unknown): void {
+  const notice = formatError(error)
+  if (isAiSeedTopicValidationError(error)) {
+    setAiSeedTopicError(notice)
+    return
+  }
+  pageNotice.value = notice
+}
+
 function clearAiCandidateProgressTimer(): void {
   if (aiCandidateProgressTimer === null) {
     return
@@ -629,6 +694,7 @@ onBeforeUnmount(() => {
 
 async function generateCandidates(): Promise<void> {
   pageNotice.value = ''
+  clearAiSeedTopicError()
   if (questionLimitReached.value) {
     pageNotice.value = t('brands.questions.messages.limitReached', {
       limit: maxQuestions.value,
@@ -662,6 +728,10 @@ async function generateCandidates(): Promise<void> {
     pageNotice.value = t('brands.questions.errors.emptyInput')
     return
   }
+  if (tooShortAiSeedTopics.value.length) {
+    setAiSeedTopicError(buildAiSeedTopicTooShortMessage(tooShortAiSeedTopics.value))
+    return
+  }
   beginAiCandidateTask('candidate')
   try {
     await previewMutations.ai.mutateAsync()
@@ -947,11 +1017,15 @@ function backToBrands(): void {
                 class="seed-topic-select"
                 mode="tags"
                 :value="aiForm.seed_topics"
+                :status="aiSeedTopicError ? 'error' : undefined"
                 :placeholder="t('brands.questions.ai.seedTopicPlaceholder')"
                 :token-separators="aiTopicTokenSeparators"
                 :options="[]"
                 @change="(value: string[]) => updateAiSeedTopics(value)"
               />
+              <p v-if="aiSeedTopicError" class="field-item__error">
+                {{ aiSeedTopicError }}
+              </p>
               <p class="field-item__hint">
                 {{ t('brands.questions.ai.seedTopicHint') }}
               </p>
@@ -1284,6 +1358,13 @@ function backToBrands(): void {
   line-height: 1.6;
 }
 
+.field-item__error {
+  margin: 0;
+  color: #b42318;
+  font-size: 13px;
+  line-height: 1.6;
+}
+
 .seed-topic-select {
   width: 100%;
 }