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

test/functional/plugin/lsp/completion_spec.lua

@@ -1398,7 +1398,7 @@ describe('vim.lsp.completion: integration', function()
     exec_lua(function()
       vim.o.completeopt = 'menuone,popup'
     end)
-    create_server('dummy', completion_list, {
+    local dummy_client_id = create_server('dummy', completion_list, {
       resolve_result = {
         {
           detail = 'function',
@@ -1474,6 +1474,41 @@ describe('vim.lsp.completion: integration', function()
       {1:~                                                 }|*15
       {5:-- INSERT --}                                      |
     ]])
+
+    n.command('lua vim.lsp.buf_detach_client(0, ' .. dummy_client_id .. ')')
+    -- Server which doesn't support completionItem/resolve
+    create_server('dummy2', {
+      isIncomplete = false,
+      items = {
+        {
+          insertText = 'package main',
+          insertTextFormat = 1,
+          kind = 9,
+          label = 'package main',
+          sortText = '0001',
+        },
+        {
+          insertText = 'package ${1:name}',
+          insertTextFormat = 2,
+          kind = 9,
+          label = 'package',
+          sortText = '0002',
+        },
+      },
+    })
+    feed('<ESC>S<C-x><C-O>')
+    -- No popup shown for item without snippet
+    wait_for_pum()
+    eq(true, n.fn.complete_info({ 'selected' }).preview_bufnr == nil)
+    feed('<C-N>')
+    -- Popup shown for item with snippet
+    screen:expect([[
+      package^                                           |
+      {4:package main Module }{100:package name}{1:                  }|
+      {12:package      Module }{1:                              }|
+      {1:~                                                 }|*16
+      {5:-- INSERT --}                                      |
+    ]])
   end)
 
   it('omnifunc works without enable() #38252', function()