fix(lsp): schedule call to vim.lsp.start for async root_dir (#31998)

When `root_dir` is a function it can (and often will) call the provided
callback function in a fast API context (e.g. in the `on_exit` handler
of `vim.system`). When the callback function is executed we should
ensure that it runs vim.lsp.start on the main event loop.

runtime/lua/vim/lsp.lua

@@ -513,8 +513,10 @@ local function lsp_enable_callback(bufnr)
         ---@param root_dir string
         config.root_dir(function(root_dir)
           config.root_dir = root_dir
+          vim.schedule(function()
             start(config)
           end)
+        end)
       else
         start(config)
       end

test/functional/plugin/lsp_spec.lua

@@ -6259,13 +6259,10 @@ describe('LSP', function()
       )
     end)
 
-    it('supports a function for root_dir', function()
+    it('supports async function for root_dir', function()
       exec_lua(create_server_definition)
 
       local tmp1 = t.tmpname(true)
-
-      eq(
-        'some_dir',
       exec_lua(function()
         local server = _G._create_server({
           handlers = {
@@ -6279,17 +6276,25 @@ describe('LSP', function()
           cmd = server.cmd,
           filetypes = { 'foo' },
           root_dir = function(cb)
+            vim.system({ 'sleep', '0' }, {}, function()
               cb('some_dir')
+            end)
           end,
         })
         vim.lsp.enable('foo')
 
         vim.cmd.edit(assert(tmp1))
         vim.bo.filetype = 'foo'
+      end)
 
+      retry(nil, 1000, function()
+        eq(
+          'some_dir',
+          exec_lua(function()
             return vim.lsp.get_clients({ bufnr = vim.api.nvim_get_current_buf() })[1].root_dir
           end)
         )
       end)
     end)
   end)
+end)