Function validate_read_mask 

pub fn validate_read_mask<M: MessageFields>(
    read_mask: Option<FieldMask>,
    default: &str,
) -> Result<FieldMaskTree, RpcError>

Validate an optional request read_mask against message type M, falling back to default when the request omits one, and return it as a FieldMaskTree. Shared by every read-mask-bearing handler (get, list, and subscribe) so validation and the “no mask -> default” rule stay identical across them. default is an explicit argument because one message type can carry different per-endpoint defaults (e.g. ExecutedTransaction uses TRANSACTION for get/list but EXECUTE_TRANSACTION for execution).