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

README.md

@@ -228,7 +228,7 @@ container.
   Handlers call the chat client on the request's own goroutine, never on
   the one that delivers events, which also delivers the chat client's
   answers. When the bot stops, requests in progress get 5 seconds to
-  finish.
+  finish before the chat client is stopped.
 - **Replies**: for each text message a contact sends in a direct chat,
   the bot sends back the result, as a reply quoting the message. Group
   messages, files and the bot's own messages are ignored.