* Add value types for registry array docblocks

* Fix covariance and normalizeArray for tightened registry array types

Tightening the registry array docblocks made child overrides and the
normalizer body inconsistent with the new value types:

- Narrow the child helper property docblocks to a string list so they
  stay covariant with the parent property type.
- Match the test app component property docblocks to the parent type.
- Guard normalizeArray so the object name is treated as a string and the
  config as an array, matching the documented input shape.

* Use registry value type for child helper property docblocks

A plain string list type is too narrow: subclasses may extend the
helpers list with array config entries such as
['Url', 'Html', 'Foo' => ['some' => 'config']]. Use the same value type
as the parent property so those overrides stay covariant.