add x-default for DeepL-Auth-Key prefix in API playground (#284)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: shirgoldbird

api-reference/openapi.yaml

@@ -3043,6 +3043,7 @@ components:
         `DeepL-Auth-Key <api-key>`
       name: Authorization
       in: header
+      x-default: "DeepL-Auth-Key "
   schemas:
     ApiKeyId:
       description: API key ID. Consists of two valid GUIDs separated by a colon.