feat: add duplicate-registration guard and structured contract errors

[Enemuo-debug]

Title: feat: add duplicate-registration guard and structured contract errors

Body:
This PR implements structured error handling and a duplicate-registration guard for the creator registry contract as requested in #1.

Changes:

• Defined ContractError enum with AlreadyRegistered and NotRegistered errors.
• Updated register_creator to return Result<(), ContractError> and prevent duplicate registrations.
• Updated buy_key to return Result<u32, ContractError> and return an error if the creator is not registered.
• Added comprehensive tests in src/test.rs covering registration, duplicate registration, and key buying scenarios.
• Enabled testutils feature for soroban-sdk in dev-dependencies to support contract testing.

Closes #1

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[Chucks1093]

Fix CI @Enemuo-debug

[Enemuo-debug]

@Chucks1093

[Chucks1093]

@Enemuo-debug fix merge conflict. tag me on the tg group when you do this

[Enemuo-debug]

@Chucks1093 merge conflict fixed

[Chucks1093]

@Enemuo-debug Fix ci