Add LHA freeze regression test

[MaxGhenis]

Summary

• add a focused regression test that exercises the LHA freeze parameter in a case where the cap binds
• verify that unfreezing LHA raises both BRMA_LHA_rate and housing_benefit_entitlement
• document the current working behavior so issue The LHA freeze parameter doesn't cause any impacts #1026 can be closed confidently

Verification

• python -m pytest -q policyengine_uk/tests/test_lha_freeze.py