fix(25_Create2): guard invalid pair creation

[vuyua9]

Summary

• Reject zero-address tokens before calculating or deploying a pair.
• Reject duplicate token pairs before the CREATE2 deployment path.
• Add regression coverage for zero addresses, identical addresses, and duplicate pairs.

Validation

• node --check 25_Create2/create2test.js
• git diff --check
• Static assertions confirmed all three revert paths and that PAIR_EXISTS is checked before new Pair{salt: salt}().

The repository checkout does not provide forge, solc, or a Hardhat dependency, so Solidity compilation and execution of the Hardhat tests could not be run locally.