fix: ensure ssh cleanup happens on cmd error

I noticed in my logs that sometimes `coder ssh` doesn't gracefully disconnect from the coordinator.

The cause is the `closerStack` construct we use in that function.  It has two paths to start closing things down:

1. explicit `close()` which we do in `defer`
2. context cancellation, which happens if the cli function returns an error

sometimes the ssh remote command returns an error, and this triggers context cancellation of the `closerStack`.  That is fine in and of itself, but we still want the explicit `close()` to wait until everything is closed before returning, since that's where we do cleanup, including the graceful disconnect.  Prior to this fix the `close()` just immediately exits if another goroutine is closing the stack.  Here we add a wait until everything is done.

cli/ssh.go

@@ -876,6 +876,7 @@ type closerStack struct {
 	closed  bool
 	logger  slog.Logger
 	err     error
+	wg      sync.WaitGroup
 }
 
 func newCloserStack(ctx context.Context, logger slog.Logger) *closerStack {
@@ -893,10 +894,13 @@ func (c *closerStack) close(err error) {
 	c.Lock()
 	if c.closed {
 		c.Unlock()
+		c.wg.Wait()
 		return
 	}
 	c.closed = true
 	c.err = err
+	c.wg.Add(1)
+	defer c.wg.Done()
 	c.Unlock()
 
 	for i := len(c.closers) - 1; i >= 0; i-- {