Quality: add missing AccessDenied (and related) error variants

[josephchimebuka]

Summary

Access-control paths return errors that are missing from ContractError (e.g. AccessDenied). Add the variants and map them in bindings.

Acceptance criteria

• Access-control failures return a dedicated, documented error code
• Bindings encode the same code/name
• Negative tests assert the specific error

Where

• contracts/src/errors.rs
• contracts/src/access_control.rs
• bindings/

[Hallab7]

@Hallab7 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I would like to work on this issue

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[drips-wave]

Congratulations, @Hallab7! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

🧑‍💻 @Hallab7: 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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[grantfox-oss]

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

@Hallab7's PR #469 was approved and merged by @josephchimebuka.

🏆 @Hallab7: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (223 total points). Track your full progress on GrantFox.

👏 Great work, @Hallab7! Keep contributing to TevaLabs.