Surface cache-read and cache-creation input tokens (plus the
prompt_tokens_include_cached_segments flag) in webhook usage responses,
so callers can distinguish cached from non-cached input tokens.
The data already flowed end-to-end via providers.Usage and
agent.RunResult.Usage; the two webhook envelope structs (webhookLLMUsage
for the sync LLM webhook, callbackUsage for the async callback payload)
copied only 3 of the fields, dropping the cache data. Add the 3 cache
fields (mirroring providers.Usage JSON tags exactly, all omitempty) and
populate them. The async callbackUsage mapping is extracted into a pure
newCallbackUsage helper for unit testing.
Additive and non-breaking: omitempty keeps responses without caching
byte-identical. No schema change.
thotam
f87cdaf616