Output descriptor differences when ValidateStandardMessageType fails.

Under normal circumstances this check is never expected to fail. Thus when it
does, it is valuable to know why exactly and not just the proto type.

PiperOrigin-RevId: 669265300

internal/proto_util.h

@@ -15,6 +15,8 @@
 #ifndef THIRD_PARTY_CEL_CPP_INTERNAL_PROTO_UTIL_H_
 #define THIRD_PARTY_CEL_CPP_INTERNAL_PROTO_UTIL_H_
 
+#include <string>
+
 #include "google/protobuf/descriptor.pb.h"
 #include "google/protobuf/util/message_differencer.h"
 #include "absl/status/status.h"
@@ -55,6 +57,8 @@ absl::Status ValidateStandardMessageType(
     descriptor_from_pool->CopyTo(&descriptor_from_pool_proto);
 
     google::protobuf::util::MessageDifferencer descriptor_differencer;
+    std::string differences;
+    descriptor_differencer.ReportDifferencesToString(&differences);
     // The json_name is a compiler detail and does not change the message
     // content. It can differ, e.g., between C++ and Go compilers. Hence ignore.
     const google::protobuf::FieldDescriptor* json_name_field_desc =
@@ -67,8 +71,8 @@ absl::Status ValidateStandardMessageType(
                                         descriptor_from_pool_proto)) {
       return absl::FailedPreconditionError(absl::StrFormat(
           "The descriptor for '%s' in the descriptor pool differs from the "
-          "compiled-in generated version",
-          descriptor->full_name()));
+          "compiled-in generated version as follows: %s",
+          descriptor->full_name(), differences));
     }
   } else {
     // Lite runtime. Just verify the message exists.