feat: Add Custom Errors for Signer Management and Signature Validation in MultiSigContractV2

[SynnekOG]

Summary

This PR enhances MultiSigContractV2 by introducing new custom errors related to signer management and signature validation. These errors strengthen the security, auditability, and maintainability of multisignature operations, while improving gas efficiency compared to string-based reverts.

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Changes

src/MultiSigContractV2.sol

• Added 4 new custom errors:

  • MultiSigContract__SignerAlreadyExists
    Raised when attempting to add a signer that is already part of the multisig group.
  • MultiSigContract__SignerNotFound
    Thrown when an operation references a signer that does not exist in the multisig group.
  • MultiSigContract__InsufficientSignatures
    Raised if a transaction does not have enough valid signatures to meet the quorum requirement.
  • MultiSigContract__InvalidAddress
    Prevents use of invalid or zero addresses in critical signer-related operations.

• Ran forge fmt to apply consistent formatting across the contract.

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Motivation

• Strengthen signer lifecycle management by clearly handling duplicate or missing signers.
• Prevent execution of under-signed transactions, reinforcing quorum requirements.
• Enhance developer experience with explicit error messages that simplify debugging and auditing.
• Reduce gas costs by replacing revert strings with efficient custom errors.

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Next Steps

• Integrate these errors into signer management functions (addSigner, removeSigner, replaceSigner) and transaction execution logic.
• Expand test coverage to validate each new error path (e.g., duplicate signers, missing signers, insufficient quorum).
• Update Natspec documentation to reflect new error semantics for external auditors and developers.

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