fix(coreapi): honor exchange expires_in when caching exchanged tokens

storeToken always used the fixed 4m cachedTokenTTL and discarded the expires_in PostOAuthToken returns. A server token shorter than 4m could be served stale, producing a 401 the cache assumed impossible. Thread expires_in through and cache for min(expires_in - buffer, cachedTokenTTL), declining to cache lifetimes shorter than the buffer.