sumTokens read the Agent SDK's per-model `modelUsage` entries with
snake_case field names, but the SDK reports those per-model entries in
camelCase (`inputTokens`, `cacheReadInputTokens`, ...). Every token
component therefore fell through to zero, silently zeroing the
cost-equivalent-token headline metric — while `total_cost_usd` and
`num_turns` (top-level snake_case) kept working and masked it.
Read `modelUsage` with the correct camelCase fields, keeping the
snake_case aggregate `usage` as the fallback. Export `sumTokens` and add
a regression test covering both the per-model camelCase sum (folding in
the auxiliary model) and the snake_case fallback, so a future SDK
field-casing drift fails a test instead of producing zero-token samples.
