Demonstrates §9.6 — per-currency cost budgets with BUDGET_EXHAUSTED
surfaced via tool_result.body.error.
python examples/cost_budget/server.py # terminal 1
python examples/cost_budget/client.py # terminal 2
Advertised features: ["cost.budget"]. Client success: one
BUDGET_EXHAUSTED observed in a tool_result; the
cost.budget.remaining snapshots are monotone non-increasing.