fix: prevent duplicate LLM stream snapshots

mentor_agent.py

@@ -10,6 +10,8 @@ from dataclasses import dataclass
 from pathlib import Path
 from typing import Any
 
+from llm_stream import OpenAIStreamAccumulator
+
 
 class MentorAgentError(RuntimeError):
     pass
@@ -211,6 +213,7 @@ def stream_with_mentor(
     try:
         with urllib.request.urlopen(request, timeout=timeout) as response:
             yielded = False
+            accumulator = OpenAIStreamAccumulator()
             for raw_line in response:
                 line = raw_line.decode("utf-8", errors="replace").strip()
                 if not line or line.startswith(":"):
@@ -227,9 +230,7 @@ def stream_with_mentor(
                 if not choices:
                     continue
                 choice = choices[0] or {}
-                content = (choice.get("delta") or {}).get("content")
-                if content is None:
-                    content = (choice.get("message") or {}).get("content")
+                content = accumulator.feed(choice)
                 if content:
                     yielded = True
                     yield str(content)