fix(resources): clarify build server hosting restrictions (#10961)

app/Http/Controllers/Api/ServersController.php

@@ -736,6 +736,13 @@ class ServersController extends Controller
             ], 422);
         }
 
+        if ($request->boolean('is_build_server') && ! $server->isBuildServer() && ! $server->isEmpty()) {
+            return response()->json([
+                'message' => 'Validation failed.',
+                'errors' => ['is_build_server' => ['A server with existing resources cannot be configured as a build server.']],
+            ], 422);
+        }
+
         $server->update($updateFields);
         if ($request->has('is_build_server')) {
             $server->settings()->update([