Run the UK contract's household route when country imports are skipped

[MaxGhenis]

Fixes #645

What was wrong

scripts/check_uk_candidate.py sets POLICYENGINE_SKIP_COUNTRY_IMPORTS=1 at startup, which makes policyengine/__init__.py bind policyengine.uk = None. The contract's _WrapperHouseholdRoute._call reached the wrapper as pe.uk.calculate_household(...), so in the gates run every wrapper.household row raised AttributeError: 'NoneType' object has no attribute 'calculate_household'. Candidate 2.134.5-1.58.2 (#639) was rejected on exactly those 16 rows (15 errors, plus unknown_path's rejection check failing on the same error). No UK candidate could qualify through certify-uk-release.

Change

• The route imports policyengine.tax_benefit_models.uk directly. It exposes the same calculate_household that policyengine.uk binds, and depends only on policyengine_uk being installed, not on the skip flag.
• A regression test runs evaluate_case(april_rate_change, "wrapper.household") in a subprocess with the flag set. It first asserts policyengine.uk is None, so it exercises the failing condition, then requires no error rows.

Verification (local, Python 3.13, locked env: policyengine-uk 2.130.1)

• The new test fails on main's route code with the production error ('NoneType' object has no attribute 'calculate_household') and passes with the fix (23 s).
• tests/test_uk_budget_reform_contract.py in full: see the comment below once it finishes.
• ruff format --check and ruff check pass on both changed files. Repo-wide ruff check exits 1 on main too with the local ruff, so that isn't from this change.

Invariant

For any caller environment, a wrapper.household row's status depends on the model's answer, not on whether POLICYENGINE_SKIP_COUNTRY_IMPORTS was set before policyengine was imported.

After merge

The next scheduled certify-uk-release run qualifies the newest pe-uk (2.134.7) on the fixed contract. Candidates #639 and #644 were staged on the old code, and the catch-up should supersede them.

🤖 Generated with Claude Code

[MaxGhenis]

Local verification at e820bef (Python 3.13, locked env: policyengine-uk 2.130.1): tests/test_uk_budget_reform_contract.py in full gives 122 passed in 4 min 3 s. The new test fails on main's route code with 'NoneType' object has no attribute 'calculate_household' and passes with the fix.

[MaxGhenis]

Merged main (#643) at 7fa30ea. The only conflict was both PRs appending tests at the end of tests/test_uk_budget_reform_contract.py, and both blocks are kept unchanged. The merged file gives 136 passed locally (3 min 22 s). The route change (e820bef) is untouched.

[MaxGhenis]

Closing as a duplicate of #651, which W1 (owner of the contract runner) opened for the same root cause, with an equivalent fix and a stricter test (household rows must pass). This PR's CI was green at 7fa30ea (Test 3.11–3.14), and its local runs (136 passed on the merged file; the new test failed on main with the production error) corroborate #651's diagnosis. Issue #645 is the same bug as #650.