Maintainer fix on stage (browser-test catch): Prism.highlightElement()
propagates the language-* class onto the parent <pre>, so previewing a
.css file then a .txt file rendered the plain text with CSS grammar.
Strip any stale language-* from #previewCode before each render and only
call highlightElement when a language was assigned. Adds regression test
(tests/test_issue3337_workspace_preview_highlight.py) and bumps the
_openSessionActionMenu scan window in test_1466 to cover the new
Regenerate-title action lines (#3223).
