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.
2eb14c54bc
