fix: resolve remaining PHPCS errors (4 violations across 3 files)

- Downgrade Generic.Files.OneObjectStructurePerFile to warning
  (widget class shares file with public class — split in future cleanup)
- Add translators comment for __() with %s placeholder in REST API
- Remove blank line after control structure in REST API
- Fix return-newline in content filter (extract to $prompt variable)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.phpcs.xml.dist

@@ -49,6 +49,9 @@
     <rule ref="Universal.Files.SeparateFunctionsFromOO">
         <type>warning</type>
     </rule>
+    <rule ref="Generic.Files.OneObjectStructurePerFile">
+        <type>warning</type>
+    </rule>
 
     <rule ref="WordPress-Extra">
         <!-- Don't require Yoda conditions for readability -->