Resolve #3635987 "Status messages are lost when using hx-select"

The first commit adds a Nightwatch test that fails on main. The second commit fixes it: core's htmx extension takes the status messages out of each response, shows them in the page's messages area and announces them. Htmx::messagesTarget() names another element for them. Five more Nightwatch cases cover escaping in the announcement, a request without hx-select, the override on the element and on a container, and a boosted link.

Closes #3635987