feat(knowledge): validate required fields based on plugin schema

Add business-agnostic validation for knowledge base creation:
- Backend: dynamically validate required fields from plugin's creation_schema
  and retrieval_schema, with support for show_if conditional fields
- Frontend: expose validation function from DynamicFormComponent and
  validate before KBForm submission
- Add i18n translations for validation error messages

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/src/i18n/locales/en-US.ts

@@ -928,6 +928,10 @@ const enUS = {
     engineSettingsDescription:
       'Configuration for the selected knowledge engine',
     engineSettingsReadonly: 'read-only in edit mode',
+    engineSettingsInvalid:
+      'Engine settings validation failed, please check required fields',
+    retrievalSettingsInvalid:
+      'Retrieval settings validation failed, please check required fields',
     retrievalSettings: 'Retrieval Settings',
     retrievalSettingsDescription:
       'Configure how documents are retrieved from this knowledge base',

web/src/i18n/locales/zh-Hans.ts

@@ -886,6 +886,8 @@ const zhHans = {
     engineSettings: '引擎设置',
     engineSettingsDescription: '所选知识引擎的配置',
     engineSettingsReadonly: '编辑模式下不可修改',
+    engineSettingsInvalid: '引擎设置中存在无效项，请检查必填字段',
+    retrievalSettingsInvalid: '检索设置中存在无效项，请检查必填字段',
     retrievalSettings: '检索设置',
     retrievalSettingsDescription: '配置从此知识库检索文档的方式',
     dangerZone: '危险区域',