feat(tests): add e2e integration test for proxy + agent + backend

Self-contained bash script that spins up the full stack (zentinel proxy,
image optimization agent via v2 UDS, python static backend) and verifies
request routing, non-image passthrough, and PNG serving through the proxy.

Conversion tests (WebP/AVIF) are auto-detected and skipped when response
event dispatch is not yet wired in the proxy.

Author: raffaelschneider