Keep reform stop dates through policy normalisation

[MaxGhenis]

Fixes #740

Review: GPT-6.1 Sol requested four changes in round 1; all are fixed, and round 2 approved this head (ece6e16). The review read the code; it did not execute it. CI passes.

What this changes

_normalise_period_key used to turn a legacy "start.stop" policy key into start. The wrapper then read it as an open-ended effective date, so every bounded reform sent to the API kept going past its end date. For example, {"gov.hmrc.income_tax.rates.uk[0].rate": {"2027-04-06.2028-04-05": 0.21}} also changed 2028-29 and every later year.

• Keys keep their stop dates. _normalise_period_key validates YYYY-MM-DD and YYYY-MM-DD.YYYY-MM-DD and returns them unchanged. A malformed key, or an interval that ends before it starts, raises ValueError naming the key.
• _compile_interval_policy handles a policy with any interval key. The pinned wrapper (6.2.3) rejects interval keys in dict policies, so this builds a policyengine.core.Policy whose ParameterValues carry explicit end_dates.
  • Paths are validated by the wrapper's own compile_reform, so unknown paths still raise with close-match hints.
  • Lone dates stay effective dates (end_date=None), and scalars keep the wrapper's {year}-01-01 default.
• Policies without interval keys keep going through the wrapper's own dict compilation, unchanged.
• Baseline artifacts: a current-law baseline (policy=None) keeps its deterministic id, and a reform never reuses a baseline artifact.

Review

GPT-6.1 Sol round 1 (on e307617) asked for two changes. Both are fixed in ece6e16, and round 2 is running.

1. Stage 12 aggregation handed the normalised dict straight to the wrapper, which rejects interval keys. Every path to the wrapper now goes through _runtime_policy (normalise, then compile intervals). New test: test_aggregate_stand_ins_keep_bounded_policy_end_dates.
2. Unpadded dates ("2027-4-6") are accepted again, as the wrapper's strptime always accepted them.

Invariants

• Round trip. For any accepted policy dict, reform_dict_from_parameter_values(compiled.parameter_values) gives back the same intervals (tested for UK and US).
• Unchanged otherwise. A policy without interval keys compiles exactly as before.
• Baseline reuse. Unchanged.

Full UK correctness for dated reforms also needs the wrapper and model fixes that this service's updater will pull in: PolicyEngine/policyengine.py#575 and PolicyEngine/policyengine-uk#2305. This PR makes sure the end date reaches them.

Tests

cd projects/policyengine-simulation-executor && uv run pytest tests/test_stage12_runtime.py tests/test_simulation_runtime_policy.py tests/test_baseline_artifacts.py tests/test_simulation_output_builder.py tests/test_budget_reform_contract_route.py: 143 passed, 1 skipped. The skipped test is the contract-route differential, which runs once the installed wrapper ships the contract; with the runner and policyengine-uk#2305 on the path, it passes all 9 cases.

The new test_simulation_runtime_policy.py covers:

• interval and open-ended legacy keys;
• mixed and reverse-ordered schedules;
• unknown paths, malformed keys and reversed intervals;
• scalars and policies without intervals (identical to the wrapper's compilation);
• UK and US round trips.

test_baseline_artifacts.py covers id behaviour for baselines and reforms. The test that asserted the stop date is dropped now asserts it is kept.

Part of the UK Autumn Budget 2026 CI plan (workstream W1).

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