Fix validator pipeline: python 3.12, grader POST, websockets

Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.11-slim-bookworm
+FROM python:3.12-slim-bookworm
 
 WORKDIR /app
 

openenv.yaml

@@ -79,7 +79,7 @@ endpoints:
     - path: "/tasks"
       method: "GET"
     - path: "/grader"
-      method: "GET"
+      method: "POST"
     - path: "/baseline"
       method: "GET"
   optional:

server/requirements.txt

@@ -10,3 +10,4 @@ openai>=1.54.4
 httpx>=0.27.0
 python-dotenv>=1.0.1
 pandas>=2.0.0
+websockets>=13.0