refactor(gem_hypercore): use fold for position aggregation in get_positions

[Copilot]

Replaces the mutable variable loop in get_positions with a functional fold, eliminating intermediate mut bindings.

let (positions, balance) = summaries.into_iter().fold(
    (Vec::new(), PerpetualBalance { available: 0.0, reserved: 0.0, withdrawable: 0.0 }),
    |(mut acc_pos, mut acc_bal), summary| {
        acc_pos.extend(summary.positions);
        acc_bal.available += summary.balance.available;
        acc_bal.reserved += summary.balance.reserved;
        acc_bal.withdrawable += summary.balance.withdrawable;
        (acc_pos, acc_bal)
    },
);

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