Use constants in tooling core

Author: Jesus Terrazas

libraries/microsoft-agents-a365-tooling/microsoft_agents_a365/tooling/services/mcp_tool_server_configuration_service.py

@@ -344,8 +344,8 @@ def _prepare_gateway_headers(
             Dictionary of HTTP headers.
         """
         return {
-            "Authorization": f"{Constants.Headers.BEARER_PREFIX} {auth_token}",
-            "User-Agent": RuntimeUtility.get_user_agent_header(orchestrator_name),
+            Constants.Headers.AUTHORIZATION: f"{Constants.Headers.BEARER_PREFIX} {auth_token}",
+            Constants.Headers.USER_AGENT: RuntimeUtility.get_user_agent_header(orchestrator_name),
         }
 
     async def _parse_gateway_response(