Update memory_manager.py

Author: dikshant182004

src/backend/agents/nodes/memory/memory_manager.py

@@ -385,7 +385,7 @@ def retrieve_ltm(student_id: str, problem_text: str, topic: str) -> dict:
                 "problem_summary", "final_answer", "outcome", "solve_attempts",
             ],
             num_results      = TOP_K_EPISODES,
-            filter_expression= f"@student_id:{{{student_id}}}",
+            filter_expression= f"@student_id:{{{student_id}}} @topic:{{{topic}}}"
             if topic is not None and topic != "": 
                 filter_expression += f" @topic:{{{topic}}}"
         )