Add default redirect URI for OAuth2 client registration

[yybmion]

Currently, OAuth2 client registration requires a redirect URI, and omitting it throws an exception during application startup. However, in most cases, users would use the standard pattern {baseUrl}/login/oauth2/code/{registrationId}.

Closes gh-16377

Implementation

• Modified ClientRegistration.Builder.build() method to set a default redirectUri value when
  • redirectUri is null
  • authorizationGrantType is AUTHORIZATION_CODE
  • registrationId is available

Testing

• Added a test to verify the default value is correctly applied when no redirect URI is provided
• Removed the previous test that expected an exception in this scenario
• Verified all existing tests continue to pass