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

openspec/openapi-v1.yaml

@@ -3381,7 +3381,7 @@ paths:
         content:
           application/json:
             schema:
-              $ref: "#/components/schemas/KnowledgeBaseRequest"
+              $ref: "#/components/schemas/KnowledgeBaseCreateRequest"
       responses:
         "200":
           $ref: "#/components/responses/Ok"
@@ -5705,22 +5705,40 @@ components:
 
     KnowledgeBaseRequest:
       type: object
-      required: [name]
       properties:
-        name:
+        kb_name:
           type: string
         description:
           type: string
+        emoji:
+          type: string
         embedding_provider_id:
-          type: string
+          type: [string, "null"]
         rerank_provider_id:
-          type: string
-        chunking:
-          $ref: "#/components/schemas/DynamicConfig"
-        metadata:
-          $ref: "#/components/schemas/DynamicConfig"
+          type: [string, "null"]
+        chunk_size:
+          type: integer
+        chunk_overlap:
+          type: integer
+        top_k_dense:
+          type: integer
+        top_k_sparse:
+          type: integer
+        top_m_final:
+          type: integer
       additionalProperties: false
 
+    KnowledgeBaseCreateRequest:
+      allOf:
+        - $ref: "#/components/schemas/KnowledgeBaseRequest"
+        - type: object
+          required: [kb_name, embedding_provider_id]
+          properties:
+            kb_name:
+              type: string
+            embedding_provider_id:
+              type: string
+
     KnowledgeDocumentUploadRequest:
       type: object
       required: [file]
@@ -5728,8 +5746,6 @@ components:
         file:
           type: string
           format: binary
-        parser:
-          type: string
 
     KnowledgeDocumentImportRequest:
       type: object