on_post_llm_call extracted usage via `if response is not None:`, taking the
response-object path. But post_api_request delivers `response` as a sanitized
dict (no `.usage` attribute) alongside a separate `usage` summary dict, so
`getattr(response, "usage")` was always None and token/cost data was dropped
for every gateway turn (traces showed usage 0 / cost 0).
Gate on a real `.usage` attribute so the existing usage-dict fallback is
reached. Real response objects (post_llm_call / legacy) still take the
response-object path. Adds regression tests for both paths.
9f1c16a7fb