Simplify YAML anchor pattern in devproxy-errors.yaml

[waldekmastykarz]

Summary

The devproxy-errors.yaml example file wraps YAML anchors in a definitions property, borrowing a JSON Schema/OpenAPI convention that doesn't fit YAML idioms. YAML anchors are a native language feature and don't need a container.

Current behavior

Anchors are defined under a definitions: wrapper:

definitions:
  client_error_body: &client_error_body
    message: "Client Error"
  server_error_body: &server_error_body
    message: "Server Error"

Proposed change

Move anchors to top-level keys, removing the unnecessary definitions nesting:

client_error_body: &client_error_body
  message: "Client Error"

server_error_body: &server_error_body
  message: "Server Error"

errors:
  ...

This is more idiomatic YAML while still keeping the anchors in a dedicated, visible location (not inlined on the first usage, which would be fragile). The schema already allows arbitrary top-level keys to support plugin config sections, so no schema changes are needed.