Fix provisional completion corrupting generated C# documents (#10806)

Fixes #10759

Initially when investigating this I was digging into buffer versions and
race conditions, but now that the editor fix is in, and we are self
versioned, the bug stood out much more obviously: A character offset was
being passed in to a parameter that expected a length 🤦‍♂️

Sadly none of the `RazorCustomMessageTarget` code is testable. Cohosting
will make this trivial though, as we would simply create a new
`Document` with the provisional change, and drop it on the floor when
we're finished.

Author: phil-allen-msft

src/Razor/src/Microsoft.VisualStudio.LanguageServices.Razor/LanguageClient/Endpoints/RazorCustomMessageTarget_Completion.cs

@@ -189,7 +189,7 @@ private static TextEdit BuildRevertedEdit(TextEdit provisionalTextEdit)
             revertedProvisionalTextEdit = VsLspFactory.CreateTextEdit(
                 range: VsLspFactory.CreateSingleLineRange(
                     range.Start,
-                    length: range.End.Character + provisionalTextEdit.NewText.Length),
+                    length: provisionalTextEdit.NewText.Length),
                 newText: string.Empty);
         }
         else