Function build_upstream_oauth_runtime 

pub async fn build_upstream_oauth_runtime(
    upstreams: &[UpstreamConfig],
    auth_config: &AuthConfig,
    encryption_key_raw: Option<&str>,
) -> Result<Option<UpstreamOauthRuntime>>

Build the upstream OAuth runtime for the upstreams that declare an oauth block.

Takes the upstream slice directly rather than a whole product config type so this runtime stays decoupled from the consumer’s config type: soma-auth reads only the upstream list, never anything else.