fix(lsp): buffer messages until connected to server (#28507)

`handle:write(msg)` can fail if the socket is not yet connected to the
server.

Should address https://github.com/neovim/neovim/pull/28398#issuecomment-2078152491

runtime/lua/vim/lsp/rpc.lua

@@ -645,9 +645,23 @@ function M.connect(host_or_path, port)
       or assert(uv.new_tcp(), 'Could not create new TCP socket')
     )
     local closing = false
+    -- Connect returns a PublicClient synchronously so the caller
+    -- can immediately send messages before the connection is established
+    -- -> Need to buffer them until that happens
+    local connected = false
+    -- size should be enough because the client can't really do anything until initialization is done
+    -- which required a response from the server - implying the connection got established
+    local msgbuf = vim.ringbuf(10)
     local transport = {
       write = function(msg)
-        handle:write(msg)
+        if connected then
+          local _, err = handle:write(msg)
+          if err and not closing then
+            log.error('Error on handle:write: %q', err)
+          end
+        else
+          msgbuf:push(msg)
+        end
       end,
       is_closing = function()
         return closing
@@ -679,6 +693,10 @@ function M.connect(host_or_path, port)
       handle:read_start(M.create_read_loop(handle_body, transport.terminate, function(read_err)
         client:on_error(M.client_errors.READ_ERROR, read_err)
       end))
+      connected = true
+      for msg in msgbuf do
+        handle:write(msg)
+      end
     end
     if port == nil then
       handle:connect(host_or_path, on_connect)