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.

索引モデルが issue の本文から書いたものです。