Add workflow to verify examples without deployment

[bnusunny]

Summary

Adds a new examples.yaml workflow that validates and tests examples locally without deploying to AWS.

Jobs

• validate — runs sam validate --lint on all 41 example templates
• build-layer — builds the x86_64 adapter layer from source
• test-image (8 examples) — builds local adapter Docker image, sam build, sam local start-api, HTTP health check
• test-zip (9 examples) — patches templates to use locally built layer, sam build, sam local start-api, HTTP health check

Details

• Triggers on PRs touching examples/** and via workflow_dispatch
• Uses --container-env-vars to set PORT=8000 avoiding conflict with SAM's RIE on port 8080
• Excluded from local API test: examples that hardcode port 8080 (nginx, php, flask, aspnet-mvc), need third-party layers (bun, nginx-zip, php-zip), or require arm64 (javalin, rust-actix-web, rust-axum)