feat(core): add error and error_description fields to responses

[halvaradop]

Description

This pull request adds support for the error and error_description properties to standardize error handling in accordance with the OAuth 2.0 RFC specification.
These properties define errors generated either by the application or by the third-party authorization service.
The implementation follows the RFC 6749, Section 5.2 guidelines for error responses from the authorization and access token endpoints.

In this update, errors are returned as JSON objects in the response body, containing the fields error and error_description.
The supported error types include:

• invalid_request
• invalid_client
• invalid_grant
• unauthorized_client
• unsupported_grant_type
• invalid_scope

This pull request primarily focuses on handling invalid_request and access_denied, which are among the most common errors returned by third-party OAuth services.

Additionally, some parts of the codebase were refactored to integrate this new feature.
New tests were added, and existing tests were updated to ensure full compatibility with the latest implementation.

References for Future Error Handling Implementations

• Authorization Error Responses
• Access Token Error Responses
• Issue Access Token Error Responses
• OAuth Extensions Error Registry