Invoking completions in the middle of an attribute and applying them results in unwanted `="$0"`

[DoctorKrolic]

Version used: https://github.com/dotnet/razor-tooling/commit/a25397d00f840c2b3276c6de7b011d29093e2880

To reproduce:

1. Create ASP.NET Core Web App (Razor pages)
2. Go to Index.cshtml
3. Insert <form action=""></form> at the end of the file
4. Invoke completions in the middle of action

Expected behavior: All completions effect attribute name, but doesn't add ="$0" because it is already specified

Actual behavior: First 3 items produce different results: First item works as expected, second even doesn't recognize snippet markup, third insert ="$0" with correct snippet behaviour, but it should just change action to asp-action