feat: add unit tests ans some description.

[philipphombergersap]

What this PR does / why we need it:

Adds focused unit tests to ensure core helper behavior is correct and stable:

- internal/helmutil/restgetter.go:
  - Validates REST config copy semantics (no mutation of base).
  - Verifies namespace propagation via RawKubeConfigLoader.
  - Ensures discovery client and REST mapper can be constructed.

- multicluster/secretprovider/provider.go:
  - Ensures clusters can be created from kubeconfig Secrets (default and namespaced keys).
  - Verifies Provider caching and List behavior.
  - Covers error case for missing kubeconfig key.

These tests improve confidence in helm integration and multicluster secret resolution without requiring a live cluster.

Special notes for your reviewer:

Release note:

add unit tests ans some description.