fix(lsp): check if buffer was detached in on_init callback (#28942)

Co-authored-by: Jongwook Choi <wookayin@gmail.com>
(cherry picked from commit af200c10cf)

Co-authored-by: Ilia Choly <ilia.choly@gmail.com>

test/functional/plugin/lsp/testutil.lua

@@ -39,8 +39,7 @@ M.create_server_definition = [[
         })
         local handler = handlers[method]
         if handler then
-          local response, err = handler(method, params)
-          callback(err, response)
+          handler(method, params, callback)
         elseif method == 'initialize' then
           callback(nil, {
             capabilities = opts.capabilities or {}