ci: simplify integration test

[maxday]

Summary

Replace the legacy CodeBuild-based CI with a GitHub Actions workflow that runs unit tests in containers
and integration tests against real Lambda functions deployed as OCI images.

• Remove all ci/codebuild/ infrastructure, vendored gtest, and old integration test harness (~27k lines
  deleted)
• Add a new integration-tests.yml workflow with a matrix of 5 OS targets (al2023, al2023-arm, ubuntu,
  alpine, arch)
• Unit tests run in native containers; integration tests build Docker images, push to ECR, deploy as
  Lambda functions, invoke them, and assert against snapshot files

Changes

• .github/workflows/tests.yml new workflow: unit-test job (container matrix) +
  integration-test job (build → ECR push → Lambda deploy → invoke → assert → cleanup)
• ci/integ/ new helper scripts: install-deps.sh, unit-test.sh, invoke.sh, assert.sh, and 4
  Dockerfiles (al2023, alpine, arch, ubuntu)
• tests/ replace vendored gtest + integration tests with a snapshot-based approach using
  lambda_function.cpp multi-handler binary
• ci/codebuild/, ci/docker/, ci/update-images.sh, ci/README.md deleted (superseded by GHA)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[maxday]

closing in favor or a PR not from a fork as it can't access the secrets