Add more scenarios to CSharpCodeActions

Outlining VSI tests

While investigating #17570, I discovered that we would sometimes create
"deferred" Roslyn projects for .NET Core projects that just hadn't been
asynchronously created yet. In *theory* that did no functional harm - they would
just get closed and converted to real projects when the project system created
the real project, but might as well not even bother creating them.

While I'm here, clean up PR feedback from #17335.

Also, update ProjectDisplayName to do what the docs say it does for Venus files.

Partially addresses #18382.