refactor(llm): drop Usage.totalInput / totalOutput helpers

The additive contract delivers value at the mapper boundary — every
field is non-overlapping and non-negative, so any caller summing
arbitrary subsets is correct by construction. Two-line helpers that
just sum three or two known fields add API surface without paying for
themselves, and there are no in-tree consumers today. If v2 wants them
at integration time, the right place is a getter on the `Schema.Class`
(matching the `LLMResponse.text` / `reasoning` / `toolCalls` pattern in
the same file), not a static namespace helper.