Implement a simplified NASDAQ market data book for a single symbol. The system receives callbacks for order insertions, modifications, and cancellations. Each order is defined by an ID, side ('buy' or 'sell'), price, and quantity. The core requirement is to implement a get_price_level(side, level_index) function that returns the price and the aggregated quantity for a specific price level. Level 0 represents the best price: the highest price for buy orders and the lowest price for sell orders.

Function Signature:

class OrderBook:
    def on_order_insert(self, order_id: int, side: str, price: int, qty: int) -> None:
        pass

    def on_order_modify(self, order_id: int, price: int, qty: int) -> None:
        pass

    def on_order_cancel(self, order_id: int) -> None:
        pass

    def get_price_level(self, side: str, level_index: int) -> tuple[int, int] | None:
        pass

Constraints:

• side is either 'buy' or 'sell'.
• Order IDs are unique for active orders.
• Modifications can alter both price and quantity.
• Canceling an unknown order should be handled gracefully.
• If a requested price level does not exist, return None.

Examples:

1. Insert buy order ID 1 (price 100, qty 5); insert buy order ID 2 (price 101, qty 3). get_price_level('buy', 0) should return (101, 3).
2. Insert sell order ID 3 (price 105, qty 2); insert sell order ID 4 (price 105, qty 7). get_price_level('sell', 0) should return (105, 9).