Implement unit tests for client credentials grant and storage mock

[Mehrn0ush]

• Added unit tests for , covering scenarios like invalid grant types, invalid client credentials, successful token issuance, and token issuance failures.
• Added unit tests for to simulate client storage for client credentials validation.
• Implemented a custom deserializer for in to handle values as empty strings, ensuring smooth deserialization.
• Adjusted the struct to use a custom deserializer for .
• Updated related functions and tests to check for empty strings instead of for .
• All tests passing.