Stringify displayId in auto_complete payload for APIv3 parity

The auto_complete endpoint exposed `displayId` as the bare
`work_package.display_id`, which is an Integer in classic mode and a
String in semantic mode. APIv3 already coerces this field on the work
package representer (and on its `:children` and `:ancestors` link
collections), so the auto_complete payload was the lone outlier.

The single CKEditor consumer interpolates the value into a string, so
this change is purely a consistency fix at the JSON boundary; nothing
behavioural depends on the wire type.

app/controllers/work_packages/auto_completes_controller.rb

@@ -60,10 +60,11 @@ class WorkPackages::AutoCompletesController < ApplicationController
       # `displayId` collapses to the numeric id in classic mode and to the
       # semantic identifier in semantic mode. The CKEditor mention plugin
       # reads it to insert `#PROJ-7` (or `#1234`) into the markdown source
-      # and to build the mention's link URL.
+      # and to build the mention's link URL. Stringified for parity with
+      # APIv3 (`WorkPackageRepresenter` serialises `displayId` as a string).
       work_package.attributes.merge(
         "to_s" => work_package.to_s,
-        "displayId" => work_package.display_id
+        "displayId" => work_package.display_id.to_s
       )
     end
   end