[BUG] Automatically generated C# .NET core client ActuatorApi.cs missing "using" statement for the model classes

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

The C# .NET Core generated client fails to build because ActuatorApi.cs is missing "using" statement for the model classes.
Build passes once using Org.OpenAPITools.Model; is manually added to ActuatorApi.cs.

openapi-generator version

openApiGeneratorPluginVersion = '5.2.1'

OpenAPI declaration file content or url

Generation Details

Steps to reproduce

Find reprex here: https://github.com/vchen8761/open-api-generator-reprex

1. Build project in CLI using: ./gradlew clean build
2. Run additional script to generate C# Client: ./gradlew codegenSdkCSharpNetCore
3. Navigate to build directory: cd .\build\sdk\csharp-netcore\
4. Build dotnet client: dotnet build
5. Build fails with error CS0246
6. Add using Org.OpenAPITools.Model; to build/sdk/csharp-netcore/src/Org.OpenAPITools/Api/ActuatorApi.cs
7. Build passes

Related issues/PRs

Suggest a fix

Reproduce the failure with ./gradlew clean build and ./gradlew codegenSdkCSharpNetCore, then inspect build/sdk/csharp-netcore/src/Org.OpenAPITools/Api/ActuatorApi.cs. Confirm the generated client fails with CS0246 and that adding the reported model namespace makes dotnet build pass; done means generation produces a buildable client without that manual edit.