fix: use HTTPBIN_URL env var for CI, fall back to testcontainers locally

[sungdark]

Summary

Fixes lihaoyi/test#1219 - requests.RequestTests timing out connecting to external httpbin.org.

Problem

Tests were failing in CI with timeouts connecting to https://httpbin.org. The kennethreitz/httpbin Docker image via testcontainers may not start reliably in GitHub Actions without DinD configuration.

Solution

HttpbinTestSuite now checks for HTTPBIN_URL environment variable first:

• CI: Set HTTPBIN_URL=http://localhost:8080 via GitHub Actions service container
• Local dev: Falls back to testcontainers Docker container (existing behavior)

CI Change Needed

Add to .github/workflows/actions.yml:

jobs:
  test:
    runs-on: ubuntu-latest
    services:
      httpbin:
        image: kennethreitz/httpbin:latest
        ports:
          - 8080:80
    steps:
      - uses: actions/checkout@v4
      - name: Run tests
        run: ./mill -i __.test
        env:
          HTTPBIN_URL: "http://localhost:8080"

Testing

• Local: ./mill -i __.test (uses testcontainers automatically)
• CI: Set HTTPBIN_URL env var to use the service container