feat: support {:array, Ash.Type.Atom} with one_of constraints in TypeMapper

[Munksgaard]

Adds a new clause to maybe_add_enum_constraint/2 that handles {:array, Ash.Type.Atom} with nested items: [one_of: [...]] constraints.

Previously, only plain Ash.Type.Atom fields with one_of were converted to {:in, string_values}. Array-of-atom fields like:

field :document_types, {:array, :atom},
  constraints: [items: [one_of: [:individual_policy, :multi_policy, :overview]]]

Would produce {:list, :atom} instead of {:list, {:in, ["individual_policy", "multi_policy", "overview"]}}.

This matters for structured LLM output schemas — without the enum constraint, the JSON schema lacks the allowed values and the model can return arbitrary strings.