Add `AnchorKitError::cache_expired` and `cache_not_found` named constructors

[Haroldwonder]

Bug Report

ErrorCode::CacheExpired (48) and ErrorCode::CacheNotFound (49) exist in the enum but have no corresponding named constructors on AnchorKitError, unlike all other error codes.

Expected Behavior

AnchorKitError::cache_expired()
AnchorKitError::cache_not_found()

Acceptance Criteria

• Both named constructors added
• Covered in test_named_constructors

[Venrable18]

@Venrable18 has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Hi! I’m a full-stack developer with experience in smart contract and frontend integration.. I’d love to take this on—I’m ready to deliver a clean, well-tested solution that aligns with your project standards. Mind assigning this to me?

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Venrable18 to this issue.

[drips-wave]

Congratulations, @Venrable18! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

🧑‍💻 @Venrable18: 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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