Add active_admin_import_context convention for controller-provided context

[Fivell]

Summary

Introduces a convention method active_admin_import_context that controllers can define to provide extra attributes for the import model. The returned hash is merged into the model after form params, so controller-authoritative values (e.g. parent.id, current_user.id, request.remote_ip) cannot be overridden by tampered form fields.

ActiveAdmin.register PostComment do
  belongs_to :post

  controller do
    def active_admin_import_context
      {
        post_id: parent.id,
        imported_by_id: current_user.id,
        request_ip: request.remote_ip
      }
    end
  end

  active_admin_import template_object: ActiveAdminImport::Model.new,
                      before_batch_import: ->(importer) {
                        importer.csv_lines.map! { |row| row << importer.model.post_id }
                        importer.headers.merge!(:'Post Id' => :post_id)
                      }
end

Why a convention method instead of a new Importer arg?

• Not hardcoded to one thing (current_user, parent, etc.) — open-ended hash
• Works with any auth setup (current_admin, Pundit scopes, etc.) since the user picks what to expose
• No API change on Importer#initialize
• Secure by default — context wins over form params, so nested belongs_to imports can't be bypassed by submitting a different parent id in the form

Supersedes / closes

• Closes Parent resource is made accesible through the template object when th… #137 — nested belongs_to parent access
• Closes Send to importer form additional params #184 — sending additional params to the importer for use in before_batch_import
• Closes add current_user to importer.rb #203 — exposing current_user inside callbacks

Test plan

• New end-to-end Capybara specs assert persisted state:
  • request.remote_ip flows through context → stored on Post records
  • Nested PostComment imports get post_id: parent.id automatically
• Full spec suite passes (56 examples, 0 failures)