Why: A provider may report prompt and completion tokens while leaving totalTokens zero; the reserve must not treat that usage as free.
Evidence: Existing budgetedStream falls back to prompt plus completion, while subRun usage accounting currently does not; the plan adds a focused regression.
Impact: Clarify the reviewed implementation scope without changing product code or authorizing a model sample.
Refs: Agent-System-Documents#95; Agent-System-Dev#512
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