test: cover StdSecp256k1 helpers - #923

GitHub issued no OIDC token to this pull_request run (fork pull requests never receive one). Harden Runner is running with the workflow's inline egress policy instead of the StepSecurity policy store.

This job is running for a fork pull request, which receives no GitHub OIDC token. No package firewall will be installed; downloads will not be inspected or blocked.