Function build_upstream_oauth_runtime_from_parts 

pub fn build_upstream_oauth_runtime_from_parts(
    upstreams: &[UpstreamConfig],
    sqlite: SqliteStore,
    key: EncryptionKey,
    redirect_uri: String,
) -> UpstreamOauthRuntime

Assemble the runtime from pre-loaded parts.

upstreams is the upstream slice (decoupled from any product config); only the entries with an oauth block get a manager.