feat: Improve OAuth Handling and Validation

[armando-rodriguez-cko]

feat: Improve OAuth Handling and Validation

Description:

This PR enhances the OAuth authentication flow in the Checkout SDK for .NET by improving token validation, refactoring OAuth models, and adding extensive test coverage.

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Key Changes:

OAuth Enhancements:

1. New Fields in OAuthAccessToken:

   • Added TokenType property.
   • Improved IsValid() check to ensure TokenType and ExpiresIn are valid.

2. Improved Token Expiration Handling:

   • _expirationDate is now stored in UTC.
   • Added exception handling for invalid tokens.

3. Refactored OAuthServiceResponse:

   • Added TokenType property.
   • IsValid() now checks for empty values more strictly.

4. Fixed OAuth Request Data Order in OAuthSdkCredentials:

   • Moved grant_type before client_id in request payload for consistency.

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Testing Improvements:

1. New Unit Tests for OAuth Handling:

   • OAuthAccessTokenTests: Ensures tokens are correctly parsed and validated.
   • OAuthServiceResponseTests: Validates response correctness.
   • OAuthSdkCredentialsTests: Mocks HTTP responses and validates token retrieval.

2. Mocked HTTP Responses:

   • Using Moq.Protected to simulate API responses for OAuthSdkCredentials.
   • Validated Authorization header handling.

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Impact:

• More Robust Authentication: Improved validation and error handling for OAuth tokens.
• Better Test Coverage: Increased reliability through extensive unit tests.
• Refactored and Cleaned Code: More maintainable and readable OAuth logic.

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Notes:

• Ensure that all API documentation is updated to reflect the changes in OAuthAccessToken and OAuthServiceResponse.
• The new tests significantly improve SDK stability by covering edge cases.