fix: align knowledge base CRUD API contract with kb_name, canonical payload fields, and matching OpenAPI types (#9000)

Warning: Potential breaking changes in /kb related openapi.

* fix(kb): align CRUD API contract end-to-end

Unify knowledge base create/update/list with kb_name, canonical_payload,
optional list pagination with total, and matching OpenAPI plus dashboard types.

* fix: restore _model_dict references, add KnowledgeBaseCreateRequest, fix OpenAPI allOf validation

- Replace 3 remaining _model_dict() calls with payload.model_dump(exclude_none=True)
- Add KnowledgeBaseCreateRequest schema with required kb_name + embedding_provider_id
- Remove additionalProperties: false from OpenAPI allOf sub-schema to avoid validation failures
- Use Pydantic Field(alias="name") + populate_by_name=True for legacy name migration
- Delegate _canonical_kb_payload to KnowledgeBaseRequest to eliminate duplicated logic
- Add getattr default None for update_kb to prevent AttributeError
- Add validation error tests: missing kb_name, missing embedding_provider_id

* fix: address knowledge base contract follow-ups

* fix: preserve knowledge base create error shape

dashboard/src/views/knowledge-base/KBList.vue

@@ -161,7 +161,9 @@
 
             <v-select v-model="formData.embedding_provider_id" :items="embeddingProviders"
               :item-title="item => item.embedding_model || item.id" :item-value="'id'"
-              :label="t('create.embeddingModelLabel')" variant="outlined" class="mb-4" :disabled="editingKB !== null" hint="嵌入模型选择后无法修改，如需更换请创建新的知识库。" persistent-hint>
+              :label="t('create.embeddingModelLabel')" variant="outlined" class="mb-4" :disabled="editingKB !== null"
+              :rules="[v => editingKB !== null || !!v || t('create.embeddingModelRequired')]" required
+              hint="嵌入模型选择后无法修改，如需更换请创建新的知识库。" persistent-hint>
               <template #item="{ props, item }">
                 <v-list-item v-bind="props">
                   <template #subtitle>
@@ -455,7 +457,10 @@ const submitForm = async () => {
     if (editingKB.value) {
       response = await knowledgeApi.update(editingKB.value.kb_id, payload)
     } else {
-      response = await knowledgeApi.create(payload)
+      response = await knowledgeApi.create({
+        ...payload,
+        embedding_provider_id: formData.value.embedding_provider_id!
+      })
     }
 
     if (response.data.status === 'ok') {