fix: #3546698 Keep the Math CAPTCHA question visible when its title is replaced

Since #3221197 (2.0.8) the Math challenge carries its question in the captcha_response #title. Code that replaces that title, such as Webform's "Question title" setting, therefore removed the question entirely; in 2.0.7 the question lived in #field_prefix and survived the override.

When the response field's #title is no longer the generated one, render the question as a field prefix and reference it from the input's aria-describedby, so it stays visible and announced. Unmodified Math challenges render exactly as before, so existing translations of "Math question (@x + @y =)" are unaffected.

Closes #3546698