fix(transport): enforce canonical base64url in b64url_decode and reject non-string fields in parse_channel_offer (#271) - #276

Awaiting 1 independent maintainer approval(s) of current head f231b13c8afb5f98dacdbac71e1dc0a3c9046778; found 0. Branch rules and specification governance also apply.

On December 2, 2026, GitHub will restrict `pull_request_target` on public repositories by default. To continue allowing the event trigger, configure an Actions policy. Learn more: https://gh.io/securely-using-pull_request_target#default-policy-for-pull_request_target