Implement buy_ticket event state validation

[Akshola00]

Description

Ensure the event exists, is active, and hasn't passed its deadline.

Testing Requirements

Write tests for EventNotFound and EventExpired.

• Crucial: Ensure all standard GitHub CI checks pass before submitting your PR!

Links

• See contributor.md for guidelines.

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[Meshmulla]

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

hello maintainer, can i work promptly on this issue

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[drips-wave]

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

🧑‍💻 @Meshmulla: 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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