Authenticated caller identity used below transport boundaries.
@type kind() :: :api_key | :user | :system | :master
@type t() :: %LLMProxy.Actor{ api_key: map() | nil, id: String.t(), kind: kind(), limits: map(), metadata: map(), name: String.t() | nil }
@spec from_api_key(map()) :: t()
@spec master_key() :: map()
@spec system(keyword()) :: t()