A client submits a job with max_cost_usd: 0.05; the agent
reports successive cost increments until the budget reaches zero,
and the next ctx.metric(...) call fails with LeaseExceededError.
Source: ../../examples/cost_budget/.
uv run python -m examples.cost_budget.server &
uv run python -m examples.cost_budget.client- Guide: Leases — cost budgets.