Replace CodeBuild integration tests with GitHub Actions matrix workflow

Motivation

The existing integration test setup relied on AWS CodeBuild with 5 separate buildspec files, a local CodeBuild agent, and multiple shell scripts to orchestrate testing across OS distributions. This was overly complex and tightly coupled to CodeBuild infrastructure.

What changed

New GitHub Actions workflow (.github/workflows/integration-tests.yml)

• Single workflow with a matrix strategy: arch × distro_config
• Runs on both x64 (ubuntu-latest) and arm64 (ubuntu-24.04-arm) runners
• Covers Alpine, Debian, Ubuntu, and AL2023 with Ruby 3.3 and 3.4
• Downloads the RIE binary directly from GitHub releases (always latest)

New AL2023 Dockerfile (Dockerfile.echo.al2023)

• Uses public.ecr.aws/lambda/provided:al2023 as the base image

New local test script (test/integration/run-local.sh)

• Single script to run any distro/version combo locally
• Usage: ./test/integration/run-local.sh debian bookworm 3.4

Removed

• All 5 CodeBuild buildspec files (test/integration/codebuild/)
• CodeBuild local agent scripts (test/integration/codebuild-local/)
• Vendored RIE .tar.gz binaries (~10MB removed from the repo)
• Old Dockerfile.echo.amazonlinux (replaced by Dockerfile.echo.al2023)

Updated

• actions/checkout bumped to v6.0.2 (pinned to SHA)

Test matrix