fix: prevent caching during vary transition

[furan917]

When a visitor switches currency or language (or other vary object someone may set), Magento issues a new X-Magento-Vary cookie value. Varnish would strip the Set-Cookie header and cache the response regardless, meaning the browser never received the updated cookie and subsequent requests continued hitting the wrong cached variant for their old context and poisoning cache for others.

This change detects when the X-Magento-Vary value the backend wants to set differs from the one the visitor sent, marks the response uncacheable, and passes Set-Cookie through to the browser so it receives the correct vary hash. Normal requests where the vary value is unchanged are unaffected.

A VTC test covers all four scenarios: vary unchanged, vary changed, first-time visitor (no existing vary cookie), and vary present in the request with no Set-Cookie in the response.

[furan917]

I think the delay on the grace.vtc needs a slightly larger delay than 2s, the varnish:fresh is pulling in varnish v9 so probably quicker.

[toonvd]

Language switches and other things that change the vary cookie should never happen on GET or HEAD requests. It's best to refrain from trying to fix bad architecture in a widely used VCL. It would be the start of chaos.