[PavelMakarchuk]

Fixes #971

Change

• DetailedBudgetProgramOutput gains is_tax: bool | None = None. It is optional, so stored outputs without it still validate, and it is null when a row has no flag.
• build_detailed_budget copies is_tax from each program-statistics row (a numpy scalar is unwrapped; anything that isn't a bool becomes null).
• Golden OpenAPI regenerated: the only change is the new optional is_tax property on DetailedBudgetProgramOutput.

Why now

PolicyEngine/policyengine.py#724 adds a UK row for every tax and benefit in the budget totals that a reform changes. The v3 app chart decides which rows are benefits from its own BENEFIT_PROGRAMS set, so new benefit rows such as scottish_child_payment and pip would be drawn as taxes. On report 4738 (2027 UC freeze), Scottish Child Payment would show −£25m instead of +£25m. With this flag the app signs rows by is_tax (PolicyEngine/policyengine-app-v2#1451). This should deploy before a .py release with #724 reaches sim-api.

Testing

• New tests in test_simulation_output_budget.py: each program carries its flag (Universal Credit and Scottish Child Payment as benefits, income tax as a tax), and the flag stays null when rows have none.
• The builder's snapshot fixture now carries is_tax.
• Executor: 1,891 tests pass. Gateway: 268 pass, including the OpenAPI golden. Contract: 322 pass.
• ruff 0.15.22 format and check pass on the changed files. Pyright passes for every package; projects/policyengine-apis-integ could not be checked locally because it needs the generated clients, which CI builds.

🤖 Generated with Claude Code

[anth-volk]

Looks good. Passing is_tax through detailed_budget lets the v3 chart sign each row from the result itself, and results stored before this still validate because the field is optional.

One minor, optional wording point: the DetailedBudgetProgramOutput description says is_tax is None only in outputs built before the flag was recorded, but _is_tax also returns None for any row without a boolean flag (as the build_detailed_budget docstring says).

As you noted, this and policyengine-app-v2#1451 need to merge and deploy before a .py release with PolicyEngine/policyengine.py#724 reaches sim-api.