[Contract] Check for duplicate tier IDs

[Yunusabdul38]

Description: Ensure data integrity during event registration by verifying that all ticket tiers provided have unique IDs. Duplicate IDs can lead to inventory management errors. Tasks:

Add a uniqueness check for tier IDs in register_event.
Return a DuplicateTierId error if a collision is detected.
Add a test case with duplicated tier IDs.

[Chibey-max]

@Chibey-max has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

i can work on this, can you assign me this?

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Chibey-max to this issue.

[drips-wave]

Congratulations, @Chibey-max! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on March 30, 2026.

🧑‍💻 @Chibey-max: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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