Stop the chat client only after the API has stopped (closes #4)

The chat client's context no longer follows the bot's, so a stop no
longer sends it SIGTERM while the API is still finishing its requests.
The deferred stop in Run, which runs after the API's, stops it instead,
so a request in progress at a stop gets its answer rather than a 500.
The README says so.

The new test runs the whole bot with the test binary itself standing in
for simplex-chat on PATH. The stand-in holds its answer to the contacts
request until the test has told the bot to stop.

Model: opus-5-5

internal/bot/bot.go

@@ -67,7 +67,9 @@ func Run(ctx context.Context, log *slog.Logger, cfg *config.Config) error {
 
 	// Cancelling this stops the chat client; the deferred wait makes
 	// Run return only once it has exited, whatever path Run takes.
-	cliCtx, stopCLI := context.WithCancel(ctx)
+	// Cancelling ctx does not reach it, so that the API, stopped first,
+	// can finish its requests while the chat client still answers.
+	cliCtx, stopCLI := context.WithCancel(context.WithoutCancel(ctx))
 
 	cli, err := simplex.StartCLI(cliCtx, log, filepath.Join(cfg.DataDir, "simplex"),
 		DisplayName, chatPort)