Make refresh_token grant type optional in DCR handler

[gazzadownunder]

Updated the Dynamic Client Registration (DCR) handler to comply with RFC 7591 by making the refresh_token grant type optional. Previously, the handler incorrectly required both authorization_code and refresh_token grant types, which was unnecessarily restrictive and non-compliant with the RFC.

Changes:

• Modified grant_types validation to only require authorization_code
• Updated error message to reflect the new requirement
• Renamed test to test_client_registration_with_authorization_code_only
• Added test for missing authorization_code (now the true error case)
• Updated test assertions to match new validation behavior

This change improves RFC 7591 compliance and provides clients with greater flexibility in their registration options.

Github-Issue: #1650