Use default cache path for TMY data when output_dir is None

[Copilot]

Running GTFSEnergyPredictor with add_hvac=True but without output_dir raised Exception: Must specify output_dir if downloading TMY data, making HVAC energy modeling impossible without specifying an output directory.

Changes

• routee/transit/thermal_energy.py: Replace the exception in add_HVAC_energy with a default cache path of ~/.cache/routee-transit/TMY when output_dir=None. Directory creation is already handled by download_tmy_files.
• tests/test_thermal_energy.py: Add test_add_HVAC_energy_no_output_dir to guard against regression; refactored shared setup into _make_mock_feed_and_trips() helper.

# Previously raised an exception — now works out of the box
predictor = GTFSEnergyPredictor(gtfs_path="data/gtfs")
results = predictor.run(add_hvac=True)