Function enforceTokenBudget

Enforces the applicable token budget for a given invocation context.

Steps:

1. Validate inputs (estimatedTokens, IRI safety).
2. SPARQL SELECT to find the most-specific budget (scope hierarchy).
3. Compute remaining = maxTokens - spentTokens (via SPARQL BIND).
4. If remaining < estimatedTokens → throw TokenBudgetExhaustedError.
5. Atomic SPARQL UPDATE (DELETE old spentTokens, INSERT new; matched by current value).
6. Verify decrement landed; throw BudgetConcurrencyError if not.
7. If newSpentTokens >= softLimitTokens → publish NATS alert.