Former title: lastRequest and lastResponse in HttpClient should be volatile

Version:

7.22.0

Code Snippet

HttpClient.java

private Response lastResponse;  // missing volatile
private Request lastRequest;  // missing volatile

Steps to Reproduce

The lastRequest and lastResponse members in HttpClient should be volatile because the class is indirectly used as a

Problem
Let's celebrate texters finishing their message sends!

Solution
Create a TEXTER_SIDEBOX implementation that upon getting to the end of sends shows a configurable (animated) gif.

Context

1. There is a new TEXTER_SIDEBOX extension framework where you can add a directory here:
   https://github.com/MoveOnOrg/Spoke/tree/main/src/integrations/texter-sideboxes
   (maybe `celebra