fix: simplify YAML anchor pattern in devproxy-errors.yaml (#1569)

Remove the unnecessary `definitions` wrapper and promote YAML anchors
to top-level keys. This is more idiomatic  anchors are a nativeYAML
language feature and don't need a JSON Schema/OpenAPI-style container.

Closes #1568

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: waldekmastykarz

DevProxy/devproxy-errors.yaml

@@ -4,12 +4,11 @@
 # It demonstrates YAML features like comments, multiline strings, and anchors.
 
 # Define reusable error body templates using YAML anchors
-definitions:
-  client_error_body: &client_error_body
-    message: "Client Error"
+client_error_body: &client_error_body
+  message: "Client Error"
 
-  server_error_body: &server_error_body
-    message: "Server Error"
+server_error_body: &server_error_body
+  message: "Server Error"
 
 errors:
   - request: