feat: identify OS-only runtimes in SDK user agents

[M-Elsaeed]

Summary

• default AWS_EXECUTION_ENV to AWS_Lambda_provided_custom in the Lambda layer bootstrap while preserving an existing value
• document the equivalent Dockerfile setting for OS-only container images, where multi-stage COPY --from cannot inherit environment metadata
• execute the real layer wrapper in an integration regression test covering both default and preserve behavior

This allows AWS SDK user agents to identify requests from OS-only Lambda runtimes, including alternative JavaScript runtimes such as Bun.

Testing

• cargo test --test integ_tests test_layer_bootstrap_sets_execution_environment
• cargo test --lib --bins --tests -- --test-threads=1 (99 passed; 4 deployment E2E tests ignored)
• cargo fmt --all -- --check
• bash -n layer/bootstrap
• cargo clippy --all-targets (passes with three pre-existing warnings in untouched files)
• cargo build

[bnusunny]

@M-Elsaeed Thanks for the PR. Can you give me more background why we want to do this in LWA instead of in the OS-only Lambda runtimes?