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/fixtures/theme_locales/objects_settings/en.yaml

@@ -9,9 +9,17 @@ en:
               label: Name
               description: "Section Name"
             links:
-              name:
-                label: Name
-                description: "Name of the link"
-              url:
-                label: URL
-                description: "URL of the link"
+              schema:
+                properties:
+                  name:
+                    label: Name
+                    description: "Name of the link"
+                  url:
+                    label: URL
+                    description: "URL of the link"
+                  child_links:
+                    schema:
+                      properties:
+                        title:
+                          label: Title
+                          description: "Title of the child link"

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",

spec/serializers/theme_objects_setting_metadata_serializer_spec.rb

@@ -21,13 +21,15 @@ RSpec.describe ThemeObjectsSettingMetadataSerializer do
       theme.save!
     end
 
-    it "should return a hash of the settings property descriptions" do
+    it "should return a hash of the settings property descriptions with schema.properties segments stripped" do
       objects_setting_locale
 
       payload = described_class.new(theme_setting[:objects_setting], root: false).as_json
 
       expect(payload[: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",
@@ -36,6 +38,8 @@ RSpec.describe ThemeObjectsSettingMetadataSerializer do
           "name.label" => "Name",
         },
       )
+
+      expect(payload[:property_descriptions]).not_to have_key("links.schema.properties.name.label")
     end
   end