feat: Introduce Custom Errors for MultiSigContractV2

[SynnekOG]

Summary

This PR enhances the MultiSigContractV2 contract with custom error handling and documentation improvements.
By introducing explicit errors and a clear Natspec section header, we improve both the readability and runtime efficiency of the contract.

Changes

• Added Natspec section header for Custom Errors in MultiSigContractV2.
• Defined new custom errors:
  • MultiSigContract__OnlyFactoryTokenContract()
    ↳ Thrown when a restricted function is called by a non-factory token contract.
  • MultiSigContract__OnlySigner()
    ↳ Thrown when a non-authorized signer attempts to act.
  • MultiSigContract__AlreadySigned()
    ↳ Thrown when a signer attempts to sign an action more than once.
• Ran forge fmt to ensure consistent formatting.

Motivation

• Gas efficiency: Custom errors are more efficient than require with revert strings.
• Clarity: Explicitly named errors improve developer experience and make reverts easier to interpret.
• Maintainability: Structured error management simplifies extending the contract with additional rules in the future.

Next Steps

• Replace existing require statements with the new custom errors.
• Extend testing to ensure each error is correctly triggered under invalid conditions.
• Add developer documentation covering error usage patterns.

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✅ Improves contract robustness, readability, and developer tooling alignment.