Fix urllib3 2.6.0+ compatibility in REST client

Replace deprecated getheaders() and getheader() methods with the
headers property API that works across urllib3 1.26.x and 2.x.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Ubuntu

generated/groundlight_openapi_client/rest.py

@@ -41,11 +41,11 @@ def __init__(self, resp):
 
     def getheaders(self):
         """Returns a dictionary of the response headers."""
-        return self.urllib3_response.getheaders()
+        return self.urllib3_response.headers
 
     def getheader(self, name, default=None):
         """Returns a given response header."""
-        return self.urllib3_response.getheader(name, default)
+        return self.urllib3_response.headers.get(name, default)
 
 
 class RESTClientObject(object):