Function resume 

pub fn resume(
    tmp_path: impl AsRef<Path>,
    url: Option<Url>,
    partial_config: PartialConfig,
    tx: Tx,
    token: CancellationToken,
)

Spawn a detached task that resumes a previously interrupted download from its .part file.

This is the one-shot form of plan_resume followed by DownloadPlan::start. url is optional. When Some, the resume resolves and validates against that URL exactly as before. When None, the task reuses the initial URL persisted in the .fd state file — the one the original download recorded (the durable initial URL, not the transient redirect/final_url). So a caller can resume purely from the .part path; redirects are re-resolved through a fresh prefetch on every resume.

If the download cannot be continued — tmp_path is not a .part file, the .fd state file is missing, the server does not support range requests, or the remote file changed — the task emits Event::ResumeError and stops without falling back to a full re-download. If tmp_path itself does not exist, the call falls back to a fresh download only when a url is available; with url = None there is nothing to fetch, so it emits ResumeError(StateError::NoUrl) instead. Likewise, when url = None but the .fd carries no resolvable URL, the call reports StateError::NoUrl.

Completion is observed the same way as download: wait for the single Event::Terminated, or drain the Rx until it disconnects.