fix: preserve nil CredentialSets in copyDCQL to pass validation

When copyDCQL copies a DCQL with no CredentialSets, it was creating
an empty slice instead of leaving it nil. This caused validation to
fail because the 'omitempty,min=1' tag on CredentialSets requires
either nil (skipped by omitempty) or at least 1 element (min=1).

This fix only creates CredentialSets if the source has elements.

Author: leifj

pkg/openid4vp/presentation_builder.go

@@ -118,8 +118,14 @@ func copyDCQL(src *DCQL) *DCQL {
 	}
 
 	dst := &DCQL{
-		Credentials:    make([]CredentialQuery, len(src.Credentials)),
-		CredentialSets: make([]CredentialSetQuery, len(src.CredentialSets)),
+		Credentials: make([]CredentialQuery, len(src.Credentials)),
+		// CredentialSets is only set if source has elements (keep it nil otherwise
+		// to pass `omitempty` validation)
+	}
+
+	// Only create CredentialSets if source has elements
+	if len(src.CredentialSets) > 0 {
+		dst.CredentialSets = make([]CredentialSetQuery, len(src.CredentialSets))
 	}
 
 	// Copy credentials