Update Mocha for Node 26 CI

Mocha 10 pulls in yargs 16, which fails under Node 26 when loaded from this type:module package. Update Mocha so the test runner uses a Node 26-compatible yargs release.\n\nCo-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: dgreif