Integration tests: WordPress and Next.js frontends with Testcontainers #398

Description

Add integration tests using Testcontainers that verify the trusted server
operates correctly against real frontend applications.

WordPress frontend

• Spin up a WordPress container with a minimal theme
• Configure the trusted server to proxy/inject into the WordPress site
• Verify:
  • HTML response contains injected <script> tags
  • /static/tsjs= endpoint serves concatenated JS bundles
  • Integration-specific attributes are rewritten in ad markup
  • GDPR consent signals propagate correctly

Next.js frontend

• Spin up a Next.js container with a minimal app
• Configure the trusted server to proxy/inject into the Next.js site
• Verify:
  • Server-side rendered pages receive script injection
  • Client-side navigation does not break after injection
  • Deferred script tags load correctly on route transitions
  • /static/tsjs= endpoint serves valid JS

Shared concerns

• Tests should be runnable locally and in CI
• Each test suite should start, exercise, and tear down containers
  automatically
• Use Testcontainers lifecycle management for setup and teardown
• Document any required Docker images or build steps in the test README

Done when

Running CI pipeline

Affected area

CI / Tooling

Additional context

No response