Return peg::ast< by value from the parsing helpers

Return peg::ast< by value from the parsing helpers: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). …and service::SubscriptionParams::query is also a std::unique ptr<peg::ast<std::string .

The task

…and `service::SubscriptionParams::query` is also a `std::unique_ptr<peg::ast<std::string>>`.

Part of FineEnvs/MiMo-V2.6-RL-harbor-code.