CI: Add Proxy integration tests (issue #84)

[s2x]

Summary

Adds comprehensive test coverage for the standalone ESI proxy server (servers/proxy/).

Changes

• proxy_test.go — 18 Go unit tests covering:

  • Proxy creation with valid/invalid backends
  • Content-Type gating (HTML processed, non-HTML passthrough)
  • ParseOnHeader mode (with/without Edge-control header)
  • Surrogate-Capability header injection (not overwritten if present)
  • Surrogate-Control response header
  • Content-Length recalculation for HTML and non-HTML
  • Error status passthrough (400, 404, 500, 502, 503)
  • Original header preservation
  • Default URL from Host header
  • Config field preservation
  • No Surrogate-Control on non-HTML
  • Backend URL and transport configuration

• test.sh — 9 E2E tests:

  1. ESI include processing (/returnEsi)
  2. ParseOnHeader bypass (no Edge-control header)
  3. ParseOnHeader active (Edge-control present)
  4. Surrogate-Control response header
  5. Non-HTML passthrough
  6. Max depth recursion limiting
  7. ESI include timeout
  8. HTTP 404 passthrough
  9. Content-Length correctness

• tests.yaml — New proxy-test CI job (after core tests, before CI gate)

• tests/server/main.go — Added Content-Type: text/html to ESI-related endpoints so proxy correctly processes HTML responses

Testing

All 18 unit tests and 9 E2E tests pass locally.