[BE] Provide script to run on console for bulk assignment

Purpose of this MR

• The current MR presents the Code Suggestions Bulk User Assignment rake task.
• The script accepts a CSV file path as a parameter and handles user assignments.
• This task is specifically designated for manual use and not automated or running within the GitLab codebase.

Local Setup and Validation Instructions

For details on setting up and validating the script locally, kindly document the file in the MR.

Related to #431619

ee/lib/duo_pro/bulk_user_assignment.rb

+# frozen_string_literal: true
+
+# Duo Pro Bulk User Assignment
+# 1. Set the `add_on_purchase` variable to point to your AddOnPurchase record
+# add_on_purchase = GitlabSubscriptions::AddOnPurchase.find_by(add_on: GitlabSubscriptions::AddOn.code_suggestions.last)
+# 2. Set the `usernames` variable to point to an array of usernames:
+#    usernames = ["user1", "user2", "user3", "user4", "user5"]
+#    If reading from a CSV file
+#    usernames =  CSV.read(FILE_PATH, headers: true).pluck('username')
+# 3. Execute the bulk assignment:
+#    DuoPro::BulkUserAssignment.new(usernames, add_on_purchase).execute
+
+# Error Messages:
+# - `User is not found`
+# - `ERROR_NO_SEATS_AVAILABLE`: No more seats are available.
+# - `ERROR_INVALID_USER_MEMBERSHIP`: User is not eligible for assignment due to being inactive, a bot, or a ghost.
+
+module DuoPro
+  class BulkUserAssignment
+    attr_reader :usernames, :add_on_purchase, :successful_assignments, :failed_assignments
+
+    def initialize(usernames, add_on_purchase)
+      @usernames = usernames
+      @add_on_purchase = add_on_purchase
+      @successful_assignments = []
+      @failed_assignments = []
+    end
+
+    def execute
+      return 'AddOn not purchased' unless add_on_purchase
+
+      process_users(usernames)
+
+      { successful_assignments: successful_assignments, failed_assignments: failed_assignments }
+    end
+
+    private
+
+    def process_users(usernames)
+      usernames.each do |username|
+        user_to_be_assigned = User.find_by_username(username)
+
+        unless user_to_be_assigned
+          log_failed_assignment("User is not found: #{username}")
+          next
+        end
+
+        result = assign(user_to_be_assigned)
+
+        if result.errors.include?("NO_SEATS_AVAILABLE")
+          log_no_seats_available(result, username)
+          break
+        end
+
+        log_result(result, username)
+      end
+    end
+
+    def assign(user)
+      ::GitlabSubscriptions::UserAddOnAssignments::SelfManaged::CreateService.new(
+        add_on_purchase: add_on_purchase,
+        user: user
+      ).execute
+    end
+
+    def log_no_seats_available(result, username)
+      log_failed_assignment("Failed to assign seat to user: #{username}, Errors: #{result.errors}")
+      log_failed_assignment("##No seats are left; users starting from @#{username} onwards were not assigned.##")
+    end
+
+    def log_successful_assignment(username)
+      successful_assignments << "User assigned: #{username}"
+    end
+
+    def log_failed_assignment(message)
+      failed_assignments << message
+    end
+
+    def log_result(result, username)
+      if result.errors.empty?
+        log_successful_assignment(username)
+      else
+        log_failed_assignment("Failed to assign seat to user: #{username}, Errors: #{result.errors}")
+      end
+    end
+  end
+end