Add getAssociated method to AbstractPersonResource

- Introduced `getAssociated` method to retrieve either `Person`, `ThirdParty`, or `null`.
- Provides a unified accessor combining `person` and `thirdParty` properties.

src/Bundle/ChillPersonBundle/Entity/Person/AbstractPersonResource.php

@@ -180,6 +180,11 @@ class AbstractPersonResource implements TrackCreationInterface, TrackUpdateInter
         return $this;
     }
 
+    public function getAssociated(): Person|ThirdParty|null
+    {
+        return $this->person ?? $this->thirdParty;
+    }
+
     /**
      * @Assert\Callback
      */