fix(lsp): highlight snippet preview when server can't completionItem/resolve (#38534)

Problem:
The snippet preview is not being highlighted by treesitter for
completion items from servers which don't support
`completionItem/resolve` (like gopls). This was broken by #38428.

Solution:
Call `update_popup_window` after updating the completion item with the
snippet preview.

I've added assertions to the `selecting an item triggers
completionItem/resolve + (snippet) preview` test case which covers the
snippet preview being shown since no tests failed when I removed the
`nvim__complete_set` call which actually populates the preview on this
codepath.

runtime/lua/vim/lsp/completion.lua

@@ -803,10 +803,11 @@ local function on_completechanged(group, bufnr)
         then
           -- Shows snippet preview in doc popup if completeopt=popup.
           local text = parse_snippet(lsp_item.insertText or lsp_item.textEdit.newText)
-          api.nvim__complete_set(
+          local windata = api.nvim__complete_set(
             data.selected,
             { info = ('```%s\n%s\n```'):format(vim.bo.filetype, text) }
           )
+          update_popup_window(windata.winid, windata.bufnr, protocol.MarkupKind.Markdown)
         end
         return
       end