[Easy] Implement credential verification logic

[Josie123-Dev]

Task Description

Add credential verification logic to contracts:

• Verify credential existence
• Check credential validity
• Validate credential status
• Return verification result
• Emit verification events

Requirements

• Existence check
• Validity check
• Status check
• Result return
• Event emission

Acceptance Criteria

• Existence is verified
• Validity is checked
• Status is validated
• Result is returned
• Events are emitted

[Mercy017]

I've shipped over 280 contributions across the ecosystem/platform with a clean review history, also contributed to this project last wave — no abandoned PRs, no back-and-forth delays. I'd like to add this issue to that list.

[grantfox-oss]

🦊 GrantFox — @Mercy017 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #78)
2. Your PR will be reviewed by the GuardZero144 maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@Mercy017's PR #133 was approved and merged by @Josie123-Dev.

🏆 @Mercy017: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (697 total points). Track your full progress on GrantFox.

👏 Great work, @Mercy017! Keep contributing to GuardZero144.