chore: refactor Batches protocol to use request models

This commit refactors the Batches protocol to use Pydantic request
models for both create_batch and list_batches methods, improving
consistency, readability, and maintainability.

- create_batch now accepts a single CreateBatchRequest parameter instead
  of individual arguments. This aligns the protocol with FastAPI’s
  request model pattern, allowing the router to pass the request object
  directly without unpacking parameters. Provider implementations now
  access fields via request.input_file_id, request.endpoint, etc.

- list_batches now accepts a single ListBatchesRequest parameter,
  replacing individual query parameters. The model includes after and
  limit fields with proper OpenAPI descriptions. FastAPI automatically
  parses query parameters into the model for GET requests, keeping
  router code clean. Provider implementations access fields via
  request.after and request.limit.

Signed-off-by: Sébastien Han <seb@redhat.com>

docs/static/stainless-llama-stack-spec.yaml

@@ -48,14 +48,18 @@ paths:
           anyOf:
           - type: string
           - type: 'null'
+          description: Optional cursor for pagination. Returns batches after this ID.
           title: After
+        description: Optional cursor for pagination. Returns batches after this ID.
       - name: limit
         in: query
         required: false
         schema:
           type: integer
+          description: Maximum number of batches to return. Defaults to 20.
           default: 20
           title: Limit
+        description: Maximum number of batches to return. Defaults to 20.
     post:
       responses:
         '200':
@@ -12690,6 +12694,22 @@ components:
       - query
       title: VectorStoreSearchRequest
       type: object
+    ListBatchesRequest:
+      description: Request model for listing batches.
+      properties:
+        after:
+          anyOf:
+          - type: string
+          - type: 'null'
+          description: Optional cursor for pagination. Returns batches after this ID.
+          nullable: true
+        limit:
+          default: 20
+          description: Maximum number of batches to return. Defaults to 20.
+          title: Limit
+          type: integer
+      title: ListBatchesRequest
+      type: object
     DialogType:
       description: Parameter type for dialog data with semantic output labels.
       properties: