Preserve nullable budget-window output fields

[anth-volk]

Summary

• Return completed budget-window status responses through JSONResponse using the full serialized payload so nullable single-year output fields remain present as null.
• Add a gateway regression assertion that completed budget-window JSON retains nullable output keys required by the generated client parser.

Verification

• uv run pytest tests/gateway/test_endpoints.py::TestBudgetWindowBatchEndpoints::test__given_batch_state__then_poll_returns_completed_response -q
• uv run ruff check src/modal/gateway/responses.py tests/gateway/test_endpoints.py
• Generated-client parse reproduction: missing nullable keys parsed as raw dict; present null keys parsed as BudgetWindowResult.