Fix default enhanced FRS Hugging Face repo URL

[MaxGhenis]

Summary

• point the default enhanced FRS dataset URL at policyengine/policyengine-uk-data-private
• update the shared ENHANCED_FRS constant to the same repo
• add a regression test covering the default Simulation() dataset URL

Closes #1533.

Verification

• pytest policyengine_uk/tests/test_deterministic_variables.py::TestDefaultDatasetUrl::test_simulation_defaults_to_private_hf_repo -q

[MaxGhenis]

This branch now also pins the default enhanced FRS dataset URL to @1.40.3 so runtime downloads use a stable HF revision rather than whatever stale cache happens to be present. That means the PR addresses both the wrong-repo bug in #1533 and the stale-cache concern in #1505.