[Contract] Refactor admin auth into helper function

[Yunusabdul38]

Description: Reduce boilerplate across administrative functions by refactoring the require_auth check for the admin address into a reusable private helper. Tasks:

Create a private helper function for admin authorization in lib.rs.
Refactor all admin-only functions to use this helper.
Ensure unit tests for authorization still pass.

[Sundriveauto]

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

pls i will love to work on this

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

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This issue has been completed by @Sundriveauto as part of the Stellar Wave Program's 3rd Wave 🥳

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