Adapt API v1 budget-window route to sim API batch backend

[anth-volk]

Summary

• stacked on Add budget-window batch economy endpoint #3387 (codex/budget-window-batch)
• replays the API v1 budget-window adapter commits on top of Add budget-window batch economy endpoint #3387
• replaces API v1 per-year ThreadPoolExecutor orchestration with a single simulation API parent batch job
• adds the budget-window batch submit/poll client paths and tests
• aligns PolicyEngine dependency pins with current master so the older Add budget-window batch economy endpoint #3387 base resolves a compatible local runtime

Scope

• Covers Stage 3 adapter work.
• Does not attempt Stage 4 cache/dedupe or full removal of the remaining budget-window dependency on reform_impacts_service / local DB tracking.

Verification

• python -m py_compile policyengine_api/libs/simulation_api_modal.py policyengine_api/services/economy_service.py tests/fixtures/libs/simulation_api_modal.py tests/fixtures/services/economy_service.py tests/unit/libs/test_simulation_api_modal.py tests/unit/services/test_economy_service.py
• uv run ruff format --check policyengine_api/libs/simulation_api_modal.py policyengine_api/services/economy_service.py tests/fixtures/libs/simulation_api_modal.py tests/fixtures/services/economy_service.py tests/to_refactor/python/test_economy_budget_window_routes.py tests/unit/libs/test_simulation_api_modal.py tests/unit/services/test_economy_service.py
• FLASK_DEBUG=1 uv run pytest tests/unit/libs/test_simulation_api_modal.py tests/unit/services/test_economy_service.py tests/unit/services/test_reform_impacts_service.py tests/unit/endpoints/test_simulation.py tests/to_refactor/python/test_economy_budget_window_routes.py