Refactor Nonce class

[grindsa]

This PR refactors the Nonce class and its test suite to improve maintainability, robustness, and clarity, while preserving all public interfaces and backward compatibility.

• Introduced a NonceRepository abstraction to encapsulate all database operations related to nonces.
• The Nonce class now delegates all database access to this repository, focusing on business logic.
• The check-and-delete operation is now more robust and clearly separated.
• Private methods have been renamed for clarity

[Copilot]

Pull request overview

This PR refactors the Nonce class by introducing a repository pattern for database operations. The changes improve separation of concerns by moving all database interactions to a dedicated NonceRepository class, while the Nonce class focuses on business logic.

• Introduced NonceRepository class to encapsulate database operations (check, delete, add)
• Renamed private methods for clarity: _new → _generate_nonce_value, _check_and_delete → _validate_and_consume_nonce
• Updated all tests to reflect the new method names and maintain coverage

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 10 comments.

File	Description
acme_srv/nonce.py	Introduces NonceRepository abstraction and refactors Nonce class to delegate database operations to the repository
test/test_nonce.py	Updates test method names, mock patches, and docstrings to align with renamed private methods

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