FEAT: Allow admin delete user's associated accounts (#29018)

This commit introduces a feature that allows an admin to delete a user's
associated account. After deletion, a log will be recorded in staff
actions.

ref=t/136675

app/controllers/admin/users_controller.rb

@@ -26,6 +26,7 @@ class Admin::UsersController < Admin::StaffController
                   disable_second_factor
                   delete_posts_batch
                   sso_record
+                  delete_associated_accounts
                 ]
 
   def index
@@ -514,6 +515,29 @@ class Admin::UsersController < Admin::StaffController
     render json: success_json
   end
 
+  def delete_associated_accounts
+    guardian.ensure_can_delete_user_associated_accounts!(@user)
+    previous_value =
+      @user
+        .user_associated_accounts
+        .select(:provider_name, :provider_uid, :info)
+        .map do |associated_account|
+          {
+            provider: associated_account.provider_name,
+            uid: associated_account.provider_uid,
+            info: associated_account.info,
+          }.to_s
+        end
+        .join(",")
+    StaffActionLogger.new(current_user).log_delete_associated_accounts(
+      @user,
+      previous_value:,
+      context: params[:context],
+    )
+    @user.user_associated_accounts.delete_all
+    render json: success_json
+  end
+
   private
 
   def fetch_user