Leave a singular association with nested attributes unwrapped

[nz]

Third of the stack, based on #53 — review that first, and this diff will shrink to its own commit once #53 merges.

Nested attributes on a singular association raise today

accepts_nested_attributes_for assigns through the public reader, and ActiveRecord asks whatever it finds there whether it is a new record. A None answers nil? but is not nil, so it reaches that question and raises:

class Book < ApplicationRecord
  include Errgonomic::Rails::ActiveRecordOptional

  belongs_to :author, optional: true
  accepts_nested_attributes_for :author
end

Book.create!(title: "Nested", author_attributes: { name: "Cixin Liu" })
# => Errgonomic::UnwrappedAccessError: undefined method `new_record?' for None

This is not new to the has_one work in #53: it is true on the released gem for any optional: true belongs_to, and it takes out build, update and destroy for the association. In the application driving this rollout it blocks six models that are otherwise ready — account.service_agreement, order_form_item.price, price.plan, cluster.space, cluster.vault, member_invitation.user.

Same shape as the encrypts exclusion

ActiveRecord needs a bare nil here, the way its encryption machinery needs the raw value behind encrypts. So this takes the shape already established for that: the association is excluded and keeps its plain reader.

The exclusion is read from nested_attributes_options, ActiveRecord's own register, rather than recorded as the macro goes past — the same way the encrypts exclusion is read from encrypted_attributes. That matters after #52: exclusions are computed when a reader is about to be wrapped, so anything that tried to append to the computed list would be silently dropped.

The concern's compromise 5 is reworded from "attributes declared with encrypts" to the general rule it now expresses: readers that ActiveRecord's own machinery reads raw. Still five compromises, not six.

Testing

rake — 30 tests, 81 assertions, and 118 doctests, all passing. The new cases cover the full nested cycle on a has_one (build, update, destroy), build on an optional: true belongs_to, and that errgonomic_optionals and errgonomic_optional_exclusions report the association as unwrapped so a conversion can be checked.