refactor: improve language normalization and date handling utilities

- Introduced a new `normalize_lang` function to standardize language codes across the application, ensuring consistent handling of user language preferences.
- Refactored date handling utilities by adding `parse_utc_iso` and `parse_utc_iso_naive` functions for better parsing of ISO 8601 date strings, enhancing timezone awareness.
- Updated various modules to utilize the new language normalization and date parsing functions, improving code clarity and maintainability.
- Enhanced error handling in date validation to raise specific `DateRangeValidationError` exceptions, providing clearer feedback on validation issues.
- Improved test coverage for date range validation and language normalization functionalities, ensuring robustness and reliability.

tests/test_handlers_commands.py

@@ -235,18 +235,15 @@ async def test_help_cmd_replies_with_help_text():
     user = _make_user()
     update = _make_update(message=message, effective_user=user)
 
-    with patch("duty_teller.handlers.commands.session_scope") as mock_scope:
-        mock_session = MagicMock()
-        mock_scope.return_value.__enter__.return_value = mock_session
-        mock_scope.return_value.__exit__.return_value = None
-        with patch(
-            "duty_teller.handlers.commands.is_admin_for_telegram_user",
-            return_value=False,
-        ):
-            with patch("duty_teller.handlers.commands.get_lang", return_value="en"):
-                with patch("duty_teller.handlers.commands.t") as mock_t:
-                    mock_t.side_effect = lambda lang, key: f"[{key}]"
-                    await help_cmd(update, MagicMock())
+    with patch(
+        "duty_teller.handlers.commands.is_admin_async",
+        new_callable=AsyncMock,
+        return_value=False,
+    ):
+        with patch("duty_teller.handlers.commands.get_lang", return_value="en"):
+            with patch("duty_teller.handlers.commands.t") as mock_t:
+                mock_t.side_effect = lambda lang, key: f"[{key}]"
+                await help_cmd(update, MagicMock())
     message.reply_text.assert_called_once()
     text = message.reply_text.call_args[0][0]
     assert "help.title" in text or "[help.title]" in text