fix: correct vm.label for user2 in Ethernaut.t.sol

[PandaLee408]

Summary

• setUp() in contracts/test/Ethernaut.t.sol was labeling user (address 0) as "User 2" instead of labeling user2 (address 1), making forge trace output misleading (two addresses both showing as related to "User 1"/mislabeled).

Fix

• Changed vm.label(user, "User 2") to vm.label(user2, "User 2").

Test plan

• forge test --match-contract TestEthernaut -vv — all 7 tests pass

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[PandaLee408]

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