[Russia] Blocking of webtunnel

Over the weekend, we received some users reports from Russia about Webtunnel bridges not working for them: https://forum.torproject.org/t/so-russia-began-blocking-webtunnel-bridges-which-is-good-actually/19538

I'm opening this ticket to investigate the issue.

Webtunnel usage: https://metrics.torproject.org/userstats-bridge-transport.html?start=2025-06-01&end=2025-06-23&transport=webtunnel

Also see this report: #40057 (comment 3214962)