fix: Allow Ctrl+C to work while waiting on OAuth flow

[jpage-godaddy]

This pkce.rs was doing a wait for a callback in such a way where SIGTERM was not working (like when hitting Ctrl+C). Fix.

[jgowdy-godaddy]

Pushed a small follow-up commit (876a0fe) on top of this branch.

Why: The async rewrite here is correct and fixes the Ctrl+C hang. In the same lines, the accept-error path also changed from fail-fast to retry-on-continue:

let (mut stream, _) = match listener.accept().await {
    Ok(conn) => conn,
    Err(_) => continue,
};

Retrying is the right call for transient, per-connection errors (e.g. ECONNABORTED). The gap is the persistent case: under fd exhaustion (EMFILE/ENFILE), accept() returns Err immediately and keeps doing so, turning the loop into a tight CPU spin that burns a core until the 120s timeout fires. The pre-PR code propagated that error instead of looping.

What changed: a 50ms backoff before retrying.

Err(_) => {
    tokio::time::sleep(Duration::from_millis(50)).await;
    continue;
}

This bounds the retry rate while still recovering if the condition clears. tokio::time::sleep is an await point, so it preserves the exact Ctrl+C cancellation behavior this PR delivers — the flow still cancels promptly mid-backoff.

Verification: full suite green on the amended branch — cargo fmt --all --check, cargo clippy --all-targets -- -D warnings, RUSTDOCFLAGS='-D warnings' cargo doc --no-deps, cargo test --all-targets (0 failures), cargo test --doc, and cargo rustdoc --lib -- -W missing-docs (0 missing-docs).

[jpage-godaddy]

Additional change from @jgowdy-godaddy looks good to me as well.