Re-enable DPoP pool server test; fix L1 user-agent marker expectation - #4143

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5, octokit/request-action@dad4362715b7fb2ddedf9772c8670824af564f0d. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/