Refactor map_account_summary_aggregate to use idiomatic iterator methods

[Copilot]

map_account_summary_aggregate used manual loops with mutable accumulators. Replaced with map/flat_map/sum iterator chains.

// Before
let mut account_value = 0.0;
for positions in positions {
    account_value += positions.margin_summary.account_value.parse::<f64>().unwrap_or(0.0);
    for position in &positions.asset_positions {
        unrealized_pnl += position.position.unrealized_pnl.parse::<f64>().unwrap_or(0.0);
    }
}

// After
let account_value: f64 = positions.iter().map(|p| p.margin_summary.account_value.parse().unwrap_or(0.0)).sum();
let unrealized_pnl: f64 = positions.iter().flat_map(|p| &p.asset_positions).map(|p| p.position.unrealized_pnl.parse().unwrap_or(0.0)).sum();

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