[Hard] Add credential testing suite

[Josie123-Dev]

Task Description

Create comprehensive credential contract testing suite:

• Unit tests for all functions
• Integration tests
• Gas optimization tests
• Security tests
• Performance tests

Requirements

• Unit test coverage
• Integration tests
• Gas tests
• Security tests
• Performance tests

Acceptance Criteria

• Unit tests pass
• Integration tests pass
• Gas is optimized
• Security is verified
• Performance is good

[nonso7]

Can i work on this issue

I am a blockchain developer with experience building decentralized applications, secure smart contracts, and user-focused Web3 products. My background spans smart contract development, protocol integration, security research, and frontend development using modern Web3 technologies. I have contributed to open-source ecosystems, built production-ready dApps, and worked on solutions involving payments, digital assets, and on-chain infrastructure. I am excited about Stellar's mission of enabling fast, low-cost, and accessible financial services, and I look forward to contributing solutions that expand financial inclusion and utility across emerging markets.

[grantfox-oss]

🦊 GrantFox — @nonso7 has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #96)
2. Your PR will be reviewed by the GuardZero144 maintainers

Good luck! Track your progress on GrantFox.