chore: improve elasticsearch_exporter maintenance path

[otjdiepluong]

Summary:

• Add or tighten focused edge-case tests or type assertions in probe_test.go, collector/ilm_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.

[sysadmind]

This is failing the DCO check. You need to sign-off your commit before we can merge.

[ArthurSens]

Hi @otjdiepluong , do you plan to solve the DCO checks?