Fix certificate lookup in validator

Re-enable the use of findSigningCertificate to ensure proper certificate validation based on X509 thumbprint. Remove hardcoding of the signingCert as first certificate in the chain and handle potential errors appropriately.

uzi_vc_validator/ura_validator.go

@@ -57,11 +57,10 @@ func (u UraValidatorImpl) Validate(jwtString string) error {
 		return err
 	}
 
-	// signingCert, err := findSigningCertificate(chainCertificates, headerValues.X509CertThumbprint)
-	signingCert := chainCertificates[0]
-	// if err != nil {
-	// 	return err
-	// }
+	signingCert, err := findSigningCertificate(chainCertificates, headerValues.X509CertThumbprint)
+	if err != nil {
+		return err
+	}
 
 	err = validateChain(signingCert, chainCertificates, u.allowUziTestCa, u.allowSelfSignedCa)
 	if err != nil {