[test] Add unit tests for WeatherRoutingTool/algorithms/data_utils.py

[adity1raut]

Description:

• The file data_utils.py contains 5 testable units (get_closest, distance, time_diffs, get_speed_from_arrival_time, and GridMixin) but has zero test coverage — no test file references it anywhere.

Acceptance criteria

• A tests/test_data_utils.py file is created
• At minimum, get_closest and get_speed_from_arrival_time have parametrized pytest tests
• Tests use pytest.fixture for reusable inputs (route arrays, datetime objects)

[adity1raut]

@MartinPontius, any suggestions on this issue, please guide me