Function await_processing 

pub async fn await_processing(
    client: &Client,
    app_id: &str,
    filter: &BuildFilter,
    config: AwaitConfig,
) -> Result<AwaitOutcome, String>

Poll BuildsApi::list_builds for app_id narrowed by filter until the newest matching build reaches a terminal processingState, or config’s timeout elapses.

list_builds returns newest-first, so the first element is the build a fresh upload produced; narrow with filter.version / filter.pre_release_version when more than one upload is in flight.

Returns Err only when the API call fails or no matching build exists at all. A build that stays PROCESSING past the timeout is a successful Ok with timed_out: true — the caller decides whether a timeout is a failure, the same way valid: false signals a binary rejection.