GPT-5 cacheCreateInputTokens accounting is not consistent with Sonnet

[dmeehan1968]

See Task.ts:226 (approx, starts with definition of isGpt5AssistantEvent) for compensation by calculating the difference in cacheInputTokens which is normally cumulative (we don't normally use the cacheInputTokens as a result).

There also seems to be an issue where the last response returns the total inputTokens for the session.