fix(frontend): make rerank model optional in agent editor save

The runtime chat pipeline (internal/application/service/chat_pipeline/rerank.go:56)
already skips rerank when rerank_model_id is empty, so it should not be
required at save time.

The editor incorrectly required rerank_model_id whenever a RAG KB was
selected, even though the field is optional. This fix removes the
validation so users can save agents without configuring a rerank model.

Fixes #1252

frontend/src/views/agent/AgentEditorModal.vue

@@ -3583,12 +3583,9 @@ const handleSave = async () => {
     return;
   }
 
-  // 校验 ReRank 模型（当需要时必填）
-  if (needsRerankModel.value && !formData.value.config.rerank_model_id) {
-    MessagePlugin.error(t('agent.editor.rerankModelRequired'));
-    currentSection.value = 'knowledge';
-    return;
-  }
+  // ReRank 模型（可选）
+  // 运行时若 rerank_model_id 为空会自动跳过 rerank，无需在保存时强制要求。
+  // 仅当用户已选择 rerank 模型时，才校验相关参数。
 
   // 过滤空推荐问题
   if (formData.value.config.suggested_prompts) {