Write unit tests for `NamespaceScopedCache`

## Description
We need to add unit tests for the `NamespaceScopedCache` implemented in https://github.com/everettraven/telescopia/blob/main/pkg/cache/namespace_cache.go

If needed, some refactoring can be done to make the code easier to test, read, understand, and maintain.

Start by reading pkg/cache/namespace_cache.go and identify the behavior and edge cases of NamespaceScopedCache. Add unit tests that cover its existing behavior and verify they pass; refactor the implementation only if needed to make those tests practical and the code easier to understand.

Written by the indexing model from the issue text.