The error codes defined in https://firebase.google.com/docs/reference/fcm/rest/v1/ErrorCode are embedded in the details section of the error response. The top-level error code (status field in the response) may take additional values such as NOT_FOUND and UNAUTHENTICATED (the so called canonical error codes):

{
  "error":{
    "code":404,
    "message":"Requested entity was not found.",
    "status":"NOT_FOUND",
    "details: [
      {
         "@type":"type.googleapis.com/google.firebase.fcm.v1.FcmError",
         "errorCode":"UNREGISTERED"
      }
    ]
  }
}

This PR updates the FCM implementation to handle these additional error codes and map them to correct client-side errors.