Add test coverage for actor availability in preparations for generic actions

[joshampton]

Problem

In version 1.8.5, prepare functions in generic actions do not have an actor even if one is set. The actor is available in the context via the run block:

defmodule Helpdesk.Ticket do
  use Ash.Resource,
    otp_app: :example,
    domain: Helpdesk,
    extensions: [AshGraphql.Resource]

  graphql do
    type :ticket

    mutations do
      action :custom, :custom
    end
  end

  actions do
    action :custom, :string do
      prepare fn input, %{actor: actor} ->
        actor |> dbg()

        Ash.ActionInput.set_context(input, %{prepare_actor: actor})
      end

      run fn _input, context ->
        %{actor: actor} = context
        %{source_context: %{prepare_actor: prepare_actor}} = context

        actor |> dbg()

        {:ok, inspect(prepare_actor)}
      end
    end
  end

  attributes do
    uuid_v7_primary_key :id
  end
end

[lib/helpdesk/ticket.ex:18: Helpdesk.Ticket.preparation_0_generated_B22ED5B3B578F28D2D69ABAB6B5758F5/2]
actor #=> nil

[lib/helpdesk/ticket.ex:27: Helpdesk.Ticket.run_0_generated_03A740289189638B3359E25173FF68B3/2]
actor #=> %Example.Actor{user: :anonymous}

[test/helpdesk/ticket_test.exs:16: Helpdesk.TicketTest."test graphql example"/1]
response #=> %{"data" => %{"custom" => "nil"}}

Fix

I believe this is already fixed in main but could use some coverage.

Test Plan

Added test "actor is set for preparations on Generic Actions" to test/plug_test.exs

Contributor checklist

Leave anything that you believe does not apply unchecked.

• I accept the AI Policy, or AI was not used in the creation of this PR.
• Bug fixes include regression tests
• Chores
• Documentation changes
• Features include unit/acceptance tests
• Refactoring
• Update dependencies