test: cover ignored tracking parameters in cached form actions

[Atroci]

Why

Ignored tracking parameters can map requests from different visitors to the same cached object. If object generation retains the first visitor query in a server-rendered form action, later visitors can submit that visitor campaign data.

Refs #855.

What

• add a generic request-derived form fixture matching the Gravity Forms boundary without adding a plugin dependency
• exercise Visitor A with gclid=A followed by Visitor B without a click ID
• assert both visitors receive the same cached HTML and a clean form action

Current trunk passes this regression. This PR pins the expected cache boundary before any production change.

Verification

• focused regression: 1/1 passed with default timeout
• full E2E suite: 7 suites, 29/29 passed under Podman with a 30-second local timeout
• pnpm typecheck
• PHP syntax and PHPCS for the fixture