Improve the error reported in the log for a NullReferenceException issue

The following error seems to be caused by spec issues, but it doesn't have enough information to trace the real issues.

```
Error: appendOadRuntimeErrors: {"type":"Raw","level":"Error","message":"Runtime Exception","time":"2025-08-06T20:28:16.228Z","groupName":"preview","extra":{"new":"https://github.com/stevemunk/azure-rest-api-specs/blob/ec36d0206de5a236b79e079611bcd25df47362cd/specification/maps/data-plane/Microsoft.Maps/Alias/preview/2.0/alias.json","old":"https://github.com/Azure/azure-rest-api-specs/blob/main/specification/maps/data-plane/Microsoft.Maps/Alias/preview/2.0/alias.json","details":"Command failed: dotnet /home/runner/work/azure-rest-api-specs/azure-rest-api-specs/node_modules/\\@azure/oad/dlls/OpenApiDiff.dll -o /tmp/oad-6bw2a3/old-resolved.json -n /tmp/oad-2X5zP2/new-resolved.json\nUnhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.\n at AutoRest.Swagger.Model.ServiceDefinition.ReferenceTrackSchemas(ServiceDefinition service) in D:\\a\\_work\\1\\s\\openapi-diff\\src\\modeler\\AutoRest.Swagger\\Model\\ServiceDefinition.cs:line 509\n at AutoRest.Swagger.Model.ServiceDefinition.Compare(ComparisonContext`1 context, ServiceDefinition previousDefinition) in D:\\a\\_work\\1\\s\\openapi-diff\\src\\modeler\\AutoRest.Swagger\\Model\\ServiceDefinition.cs:line 321\n at AutoRest.Swagger.SwaggerModeler.Compare(String fileNameOld, String swaggerOld, String fileNameNew, String swaggerNew, Settings settings) in D:\\a\\_work\\1\\s\\openapi-diff\\src\\modeler\\AutoRest.Swagger\\SwaggerModeler.cs:line 34\n at OpenApiDiff.Program.Main(String[] args) in D:\\a\\_work\\1\\s\\openapi-diff"}}
```

Pipeline run:
https://github.com/Azure/azure-rest-api-specs/actions/runs/16787747592/job/47542471322?pr=36493

Start with ServiceDefinition.cs at ReferenceTrackSchemas line 509, then follow Compare at line 321 and SwaggerModeler.cs line 34. Reproduce the failure using the OpenApiDiff.dll command and the old/new resolved files shown in the error details. Done means the log reports enough context to trace the underlying specification problem instead of only the NullReferenceException.

Written by the indexing model from the issue text.