chore: improve blackbox_exporter maintenance path

[otjdiepluong]

Summary:

• Add or tighten focused edge-case tests or type assertions in main_test.go, prober/dns_test.go related to CI, Docker, Kubernetes, build tooling, release automation; avoid docs-only changes and broad refactors.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.