[release/6.0] Handle shortened JSON file in `dotnet openapi` (#36355)

* Clean backport of e23fd047

* Handle shortened JSON file in `dotnet openapi`
  - #35767
  - an existing JSON file must be truncated
* !fixup! Address nits in changed file
  - take VS suggestions
* Stop skipping `dotnet openapi` tests
  - .NET SDKs and Visual Studio's `msbuild` avoid #32686 already
* !fixup! Address nits in `dotnet openapi` test files
  - take VS suggestions
* Extend `dotnet openapi refresh` tests
  - include regression test for #35767