FIX: support nested descriptions in object settings (#37538)

Reported here:
https://meta.discourse.org/t/labels-and-descriptions-missing-from-nested-object-settings/394685

Descriptions for nested object settings were not being displayed due to
a mismatch in locale key formatting. Stripping `.schema.properties.` so
that the locale keys from the serializer match the keys expected in the
template fixes it

Before: 
<img width="600" alt="image"
src="https://github.com/user-attachments/assets/1e0ab4c6-e6eb-478d-97bc-76f16739135b"
/>


After: 
<img width="600" alt="image"
src="https://github.com/user-attachments/assets/a9a9e64c-4efa-4807-94a1-dd853ffd9f04"
/>

spec/requests/admin/themes_controller_spec.rb

@@ -1647,6 +1647,8 @@ RSpec.describe Admin::ThemesController do
 
         expect(response.parsed_body["property_descriptions"]).to eq(
           {
+            "links.child_links.title.description" => "Title of the child link",
+            "links.child_links.title.label" => "Title",
             "links.name.description" => "Name of the link",
             "links.name.label" => "Name",
             "links.url.description" => "URL of the link",