CVE-2026-89313: JwksTrustStore.Get returns the last fetched JWKS key (err=nil) when the requested kid is absent, instead of nil

JwksTrustStore.Get assigned each parsed key to the function's named return value while refreshing the JWKS cache. When the requested kid was not present, the trailing naked return handed the caller the last key in the JWKS document together with a nil error — a confidently wrong answer instead of "not found".