chore: fix race in cron close behavior (TestAgent_WriteVSCodeConfigs) (#11243)

* chore: add unit test to excercise flake
* Implement a *fix for cron stop() before run()

This fix still has a race condition. I do not see a clean solution
without modifying the cron libary. The cron library uses a boolean
to indicate running, and that boolean needs to be set to "true"
before we call "Close()". Or "Close()" should prevent "Run()"
from doing anything.

In either case, this solves the issue for a niche unit test bug
in which the test finishes, calling Close(), before there was
an oppertunity to start the go routine. It probably isn't worth
a lot of time investment, and this fix will suffice

agent/agentscripts/agentscripts_test.go

@@ -53,6 +53,15 @@ func TestTimeout(t *testing.T) {
 	require.ErrorIs(t, runner.Execute(context.Background(), nil), agentscripts.ErrTimeout)
 }
 
+// TestCronClose exists because cron.Run() can happen after cron.Close().
+// If this happens, there used to be a deadlock.
+func TestCronClose(t *testing.T) {
+	t.Parallel()
+	runner := agentscripts.New(agentscripts.Options{})
+	runner.StartCron()
+	require.NoError(t, runner.Close(), "close runner")
+}
+
 func setup(t *testing.T, patchLogs func(ctx context.Context, req agentsdk.PatchLogs) error) *agentscripts.Runner {
 	t.Helper()
 	if patchLogs == nil {