Fixing TagHelper attribute name completion at attribute name end

[alexgav]

Summary of the changes

• Another case where we need to fixup owner returned by FindInnermostNode (that replaced previously used LocateOnwer) in RazorCompletionContext
• We were already fixing up cases for start/end element name, but we weren't for attributes
• Previously LocateOwner always gave us MarkupTextLiteral that was a child of the MarkupTagHelperAttribute, but now we get MarkupTagHelperAttribute itself from FindInnermostNode, so we need to add it to the list of exclusions where we don't need to get a parent of the owner
• Similar to what was done in Remove LocateOwner from completion scenarios #9455

Fixes:
#9728