I have been following a similar process, and am able to get it working if I use a different extension (e.g., .foo as used in the original MS source).
Perhaps this is because the .cpp extension is already defined internally by VS.

[FileExtension(".cpp")]

The language client also failed to initialise for me when I set the extension to .cs and tried to open a .cs file.
Did either of you make any progress with this?