feat: Support Roundcube 1.7 static files handling

Roundcube 1.7 forces us to go trough static.php for security. However,
this script does not exist in older Roundcube releases. Fortunately,
there's an API function available that exists in both old and new
releases and accounts for the new behaviour.

test: reset rcmail singleton between tests

Agent-Logs-Url: https://github.com/GermanCoding/Roundcube_TLS_Icon/sessions/1a348c23-50d8-4c86-9ac7-408ac7badf4a

Co-authored-by: GermanCoding <4279661+GermanCoding@users.noreply.github.com>

chore: Add extra test for new static file handling

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: GermanCoding