Add integration testing with testcontainers and Playwright

[prk-Jr]

Summary

• Add end-to-end integration testing infrastructure using testcontainers for HTTP-level tests and Playwright for browser tests against WordPress and Next.js containers
• Add Playwright browser tests verifying script injection, SPA navigation, deferred script execution, API passthrough, and form URL rewriting in a real Chromium browser
• Introduce a /health endpoint and CI workflow for running both test suites separately from the main CI

Changes

File	Change
crates/fastly/src/main.rs	Add /health GET endpoint returning 200 with "ok" body
crates/integration-tests/Cargo.toml	New crate with reqwest, testcontainers, and tokio dependencies
crates/integration-tests/tests/common/	Core test infrastructure: runtime traits, config builder, assertion helpers including assert_form_action_rewritten (9 unit tests)
crates/integration-tests/tests/environments/	Fastly/Viceroy runtime environment with dynamic port allocation
crates/integration-tests/tests/frameworks/	Framework abstractions for WordPress and Next.js with 7 standard + 5 custom scenarios
crates/integration-tests/tests/integration.rs	Matrix test runner (test_all_combinations, per-framework tests)
crates/integration-tests/fixtures/configs/	Viceroy and Fastly config templates with KV stores and secrets
crates/integration-tests/fixtures/frameworks/wordpress/	WordPress Docker image: PHP built-in server with minimal test theme and /wp-admin/ stub
crates/integration-tests/fixtures/frameworks/nextjs/	Next.js 14 Docker image: 4 pages, API routes, forms, shared navigation with hydration signal, deferred route scripts
crates/integration-tests/browser/	Playwright test suite: global setup/teardown, Docker + Viceroy infra helpers with cleanup-on-failure
crates/integration-tests/browser/tests/shared/	Cross-framework browser tests: script injection, script bundle loading
crates/integration-tests/browser/tests/nextjs/	Next.js browser tests: 4-page SPA navigation chain, deferred script execution, API passthrough, form URL rewriting
crates/integration-tests/browser/tests/wordpress/	WordPress browser tests: admin page script injection
.github/workflows/integration-tests.yml	CI workflow with parallel HTTP-level and browser test jobs, split artifact uploads for Playwright reports and traces
scripts/integration-tests.sh	Shell script for HTTP-level test orchestration (WASM build + Docker + test run)
scripts/integration-tests-browser.sh	Shell script for browser test orchestration (WASM build + Docker + Playwright install + test run)
crates/integration-tests/README.md	Full documentation: prerequisites, quick start, test scenario tables, architecture overview
INTEGRATION_TESTS_PLAN.md	Design document with architecture decisions and progress tracking
Cargo.toml	Add integration-tests to workspace members
Cargo.lock	Updated lockfile with new dependencies

Test coverage

HTTP-level (Rust + testcontainers)

Scenario	Frameworks
HtmlInjection	WordPress, Next.js
ScriptServing	WordPress, Next.js
AttributeRewriting	WordPress, Next.js
ScriptServingUnknownFile404	WordPress, Next.js
WordPressAdminInjection	WordPress
NextJsRscFlight	Next.js
NextJsServerActions	Next.js
NextJsApiRoute	Next.js
NextJsFormAction	Next.js

Browser-level (Playwright + Chromium)

Spec	Frameworks
script-injection	WordPress, Next.js
script-bundle	WordPress, Next.js
navigation (4-page SPA chain + back button + deferred route script)	Next.js
api-passthrough	Next.js
form-rewriting	Next.js
admin-injection	WordPress

Totals: 12 Next.js + 5 WordPress browser tests, 0 flaky

Known gaps

• GDPR consent propagation — the consent field exists in AuctionRequest but is not yet populated upstream. Requires implementation before it can be tested.
• Browser suite uses manual Docker orchestration — HTTP-level tests use the Rust testcontainers crate; browser tests use docker run/docker stop via Playwright's globalSetup/globalTeardown (Node.js cannot use the Rust crate).

Closes

Closes #398

Test plan

• cargo test --workspace (excluding integration-tests, which requires native target)
• cargo clippy --all-targets --all-features -- -D warnings (excluding integration-tests)
• cargo fmt --all -- --check
• JS tests: pre-existing ESM/CJS failure on main (not introduced by this branch)
• JS format: cd crates/js/lib && npm run format -- passes
• Docs format: cd docs && npm run format -- passes
• Integration-tests crate compiles for native target: cargo test -p integration-tests --target aarch64-apple-darwin --no-run
• HTTP integration tests: ./scripts/integration-tests.sh -- all pass
• Browser integration tests: ./scripts/integration-tests-browser.sh -- 17 pass, 0 fail
• Cargo clippy on integration-tests: cargo clippy --manifest-path crates/integration-tests/Cargo.toml --tests -- clean

Checklist

• Changes follow CLAUDE.md conventions
• No unwrap() in production code — use expect("should ...")
• Uses log macros (not println!)
• New code has tests (9 unit tests for assertions and config, 17 browser tests)
• No secrets or credentials committed