Microsoft.Extensions.ApiDescription.Server : After build, OpenAPI specification file isn't present in the build result

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

I try to follow the documentation [Generate OpenAPI documents at build-time](https://learn.microsoft.com/en-us/aspnet/core/fundamentals/openapi/aspnetcore-openapi?view=aspnetcore-9.0&tabs=net-cli#generate-openapi-documents-at-build-time), but the OpenAPI specification file isn't present in the build result.

I reproduce on Windows 10 and Windows 11 (I haven't tested it elsewhere).

### Expected Behavior

The OpenAPI specification file is in the build result.

### Steps To Reproduce

So the script :
```
dotnet new webapi --name MyApi --no-https --use-program-main --use-controllers
cd MyApi
dotnet add package Microsoft.Extensions.ApiDescription.Server
dotnet build
```

Then if we display the bin :
```
> dir bin\Debug\net9.0

Volume in drive C has no label.
Volume Serial Number is 60F0-FEC7

Directory of C:\t\dotnte-poc\MyApi\bin\Debug\net9.0

05/12/2024 15:50 .
05/12/2024 15:50 ..
05/12/2024 15:50 127 appsettings.Development.json
05/12/2024 15:50 151 appsettings.json
29/10/2024 14:30 192 800 Microsoft.AspNetCore.OpenApi.dll
06/08/2024 10:15 239 024 Microsoft.OpenApi.dll
05/12/2024 15:50 2 235 MyApi.deps.json
05/12/2024 15:50 8 704 MyApi.dll
05/12/2024 15:50 145 408 MyApi.exe
05/12/2024 15:50 21 348 MyApi.pdb
05/12/2024 15:50 416 MyApi.runtimeconfig.json
05/12/2024 15:50 68 MyApi.staticwebassets.endpoints.json
10 File(s) 610 281 bytes
2 Dir(s) 98 535 710 720 bytes free
```
The file `MyApi.json` is missing.

---

But the file is correctly generated and present in `obj` :
```
>dir obj

Volume in drive C has no label.
Volume Serial Number is 60F0-FEC7

Directory of C:\t\dotnte-poc\MyApi\obj

05/12/2024 15:50 .
05/12/2024 15:50 ..
05/12/2024 15:50 Debug
05/12/2024 15:50 2 982 MyApi.csproj.nuget.dgspec.json
05/12/2024 15:50 2 165 MyApi.csproj.nuget.g.props
05/12/2024 15:50 551 MyApi.csproj.nuget.g.targets
05/12/2024 15:50 1 730 MyApi.json <<<<< Here
05/12/2024 15:50 38 MyApi.OpenApiFiles.cache
05/12/2024 15:50 20 520 project.assets.json
05/12/2024 15:50 581 project.nuget.cache
7 File(s) 28 567 bytes
3 Dir(s) 98 537 242 624 bytes free
```

### Exceptions (if any)

_No response_

### .NET Version

9.0.101

### Anything else?

_No response_

Start with the documented build-time OpenAPI steps and the `dotnet new webapi`, package-add, and `dotnet build` reproduction. Compare the generated `obj/MyApi.json` with the `bin/Debug/net9.0` contents, then trace the Microsoft.Extensions.ApiDescription.Server build integration; done means the specification file is present in the build result as expected.

Written by the indexing model from the issue text.