feat: use Responses protocol for RAG preparation

backend/app/services/completions.py

@@ -48,7 +48,11 @@ def request_body(query: str, evidence: list, options: CompletionOptions):
 def parse_completion(payload: dict, context: ContextRead) -> ParsedCompletion:
     """Structural/citation checks only; a valid citation is not an entailment proof."""
     try:
-        if context.status != "prepared" or not context.evidence:
+        if (
+            not isinstance(context.completion, CompletionOptions)
+            or context.status != "prepared"
+            or not context.evidence
+        ):
             raise ValueError("context has no evidence")
         if payload["object"] != "chat.completion":
             raise ValueError("non-completion response")