Investigate current stepup-behat YubiKey verification regressions #99

After the HAProxy certificate fixes in PR #98 and the local selfservice logout redirect fix in commit 2d3ddb0, the stepup-behat workflow still fails on the current stack with 89/155 failed scenarios in run 34231412724.

Current state:

• the previous cURL error 7: Failed to connect to www.surf.nl failures are gone after overriding Stepup-SelfService logout redirects to a local SSP URL
• 88 failures are now Form field with id|name|label|value|placeholder "gateway_verify_yubikey_yubikeyInput" not found
• 1 additional failure is The following tokens are registered for your account. missing on selfservice.feature:30

Why this looks separate from PR #98:

• PR ci(stepup-behat): generate HAProxy dev certificate before compose up #98 only fixes GitHub Actions / local cert bootstrapping in devconf
• the YubiKey selector in devconf Behat contexts is already up to date (gateway_verify_yubikey_yubikeyInput)
• current Stepup-Gateway source still renders form.yubikeyInput in the YubiKey verify template
• an earlier branch run on 2026-08-06 had 153 passed / 2 failed and only the www.surf.nl logout failures, so the broad YubiKey breakage appears to be a later regression

Working hypothesis:

• the remaining failures come from an upstream application/image regression in the floating :test Stepup stack (Gateway / SelfService / GSSP interaction), not from the devconf workflow cert changes themselves

Follow-up:

• reproduce against the current :test images
• capture the actual HTML returned on the failing /verify-second-factor/{sso,sfo}/yubikey page
• inspect recent Stepup-Gateway / SelfService / GSSP changes since the last near-green run