test: add error path test for live trading get_data

- Add check for `mt5.copy_rates_from_pos` returning `None` in `live_trading/multi_bar.py:82`
- Add test in `tests/test_multi_bar.py` verifying that when `mt5.copy_rates_from_pos` returns `None`, the function returns `None` and an error is logged.

Co-authored-by: maghdam <63883156+maghdam@users.noreply.github.com>

tests/test_multi_bar.py

@@ -0,0 +1,24 @@
+import sys
+from unittest.mock import MagicMock
+# Mock out MetaTrader5 before importing our module
+sys.modules['MetaTrader5'] = MagicMock()
+
+import unittest
+from unittest.mock import patch
+import pandas as pd
+from live_trading.multi_bar import TradingApp, log_and_print
+
+class TestTradingApp(unittest.TestCase):
+    def setUp(self):
+        self.app = TradingApp(symbol="EURUSD", lot_size=0.01, magic_number=123456)
+
+    @patch("live_trading.multi_bar.mt5.copy_rates_from_pos")
+    @patch("live_trading.multi_bar.log_and_print")
+    def test_get_data_returns_none(self, mock_log, mock_copy_rates):
+        mock_copy_rates.return_value = None
+
+        # Test what happens when mt5.copy_rates_from_pos returns None
+        result = self.app.get_data("EURUSD", 100, 16408)
+
+        self.assertIsNone(result)
+        mock_log.assert_called_once_with("Could not retrieve data for EURUSD", is_error=True)