fix: implement missing _get_history_text in RagChatUseCase to fix 500 error during stream

src/core/use_cases/rag_chat_use_case.py

@@ -779,8 +779,22 @@ JSON:"""
         # Collapse multiple blank lines
         content = re.sub(r'\n{3,}', '\n\n', content)
         return content.strip()
-        past_messages = self.chat_history_db.get_history(session_id, limit=6)
-        return "".join([f"{msg.role}: {msg.content}\n" for msg in past_messages])
+
+    def _get_history_text(self, session_id: str) -> str:
+        if not session_id:
+            return ""
+        try:
+            history = self.chat_history_db.get_history(session_id, limit=6)
+            if not history:
+                return ""
+            history_lines = []
+            for msg in history:
+                role = "User" if msg.role == "user" else "Assistant"
+                history_lines.append(f"{role}: {msg.content}")
+            return "\n".join(history_lines)
+        except Exception as e:
+            logger.error(f"[RAG] Failed to fetch history: {e}")
+            return ""
     
     def _get_cache_keys(self, query: str) -> Dict[str, str]:
         """Generate cache keys — kept for backward compat, new code uses RedisAdapter methods directly."""
@@ -826,7 +840,7 @@ JSON:"""
                 return cached_result
 
         logger.info("[RAG] Cache MISS — running full RAG pipeline")
-        history_text = self._get_history_text(session_id)
+        history_text = "" if is_guest else self._get_history_text(session_id)
 
         context_text, final_sources = await self._build_context(
             request.query,