feat(stellar): add Horizon mock server and fix pool sequence resync & tests. - #1890

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: release-drafter/release-drafter@v6. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

"pull_request_target.edited" is not a known webhook name (https://developer.github.com/v3/activity/events/types/)

"pull_request_target.synchronize" is not a known webhook name (https://developer.github.com/v3/activity/events/types/)

"pull_request_target.reopened" is not a known webhook name (https://developer.github.com/v3/activity/events/types/)