Register a runtime harness

curl --request POST \
  --url https://litellm-rust.onrender.com/api/runtime-harnesses \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "api_base": "http://opencode:8080",
  "api_key": "local-opencode-key"
}
'

{
  "harnesses": [
    {
      "alias": "local-opencode",
      "api_spec": "claude_managed_agents",
      "display_name": "local-opencode",
      "api_base": "http://opencode:8080",
      "is_default": true,
      "connected": true,
      "masked_api_key": "<string>",
      "tools": [
        {}
      ]
    }
  ]
}