Publish conflicts when using agora_rtc_sdk with .NET 9 - Multiple duplicate files with same relative path

### Issue Summary
When attempting to publish a .NET 9 WPF application that uses the `agora_rtc_sdk` NuGet package (version 4.2.6.19-alpha.1), the publish process fails due to duplicate file conflicts.

### Error Message
```
Found multiple publish output files with the same relative path:
C:\Users\{User}\.nuget\packages\agora_rtc_sdk\4.2.6.19-alpha.1\build\x64\net40\agorartc.dll,
C:\Users\{User}\.nuget\packages\agora_rtc_sdk\4.2.6.19-alpha.1\build\x64\netcoreapp20\agorartc.dll,
C:\Users\{User}\.nuget\packages\agora_rtc_sdk\4.2.6.19-alpha.1\build\x64\net40\agorartc.pdb,
C:\Users\{User}\.nuget\packages\agora_rtc_sdk\4.2.6.19-alpha.1\build\x64\netcoreapp20\agorartc.pdb
```

### Root Cause
The NuGet package includes both .NET Framework 4.0 (`net40`) and .NET Core 2.0 (`netcoreapp20`) versions of the same files (`agorartc.dll` and `agorartc.pdb`). When publishing a .NET 9 application, both versions are included in the publish output, causing filename conflicts.

### Environment
- **Target Framework**: .NET 9 (net9.0-windows)
- **Project Type**: WPF Application
- **NuGet Package Version**: 4.2.6.19-alpha.1
- **Build Action**: Folder Publish
- **OS**: Windows

### Current Workaround
We've implemented a custom MSBuild target to exclude the .NET Framework 4.0 versions:

```xml

```

### Expected Behavior
The publish process should automatically select the appropriate version of the native libraries based on the target framework without requiring manual intervention or custom MSBuild targets.