feat: personalize and stream mentor chat

tests/test_account_data_boundaries.py

@@ -78,6 +78,21 @@ class AccountDataBoundaryTests(unittest.TestCase):
             self.database.delete_mentor_messages(self.first["id"], "mentor-a", "20260721"), 2
         )
 
+    def test_mentor_preferences_are_scoped_by_user(self):
+        self.database.save_mentor_preferences(
+            self.first["id"], ["mentor-b", "mentor-a"], {"mentor-b"}
+        )
+        self.database.save_mentor_preferences(
+            self.second["id"], ["mentor-a", "mentor-b"], set()
+        )
+
+        first = self.database.list_mentor_preferences(self.first["id"])
+        second = self.database.list_mentor_preferences(self.second["id"])
+        self.assertEqual([item["mentor_id"] for item in first], ["mentor-b", "mentor-a"])
+        self.assertTrue(first[0]["pinned"])
+        self.assertEqual([item["mentor_id"] for item in second], ["mentor-a", "mentor-b"])
+        self.assertFalse(any(item["pinned"] for item in second))
+
     def test_latest_data_snapshot_skips_demo_and_future_records(self):
         self.database.save_data_snapshot(
             "stock_detail", "002141:20260718", "tushare", {"marker": "real"}