Redundant positionId in Position struct

The Position struct includes a positionId field that duplicates the key used in the positions mapping:

struct Position {
    uint positionId;       // Redundant with mapping key
    address walletAddress;
    // ... other fields
}

mapping(uint => Position) public positions;  // positionId is already the key

While this might seem convenient for frontend integration or event emission, it:

1. Wastes gas by storing the same data twice.
2. Requires maintaining consistency between mapping key and struct field.
3. Increases contract deployment cost.

Remove the redundant positionId from the struct and use the mapping key when needed.