feat: support Ruby 4.0

[zarirhamza]

What does this PR do?

Adds support for Ruby 4.0, following the same pattern as #118 (Ruby 3.4).

AWS Lambda added the ruby4.0 managed runtime in April 2026 (announcement). Ruby 4.0 is the latest LTS release (supported through March 2029) and the base image public.ecr.aws/lambda/ruby:4.0 is available for both amd64 and arm64.

Adds two new published layers per architecture:

• Datadog-Ruby4-0
• Datadog-Ruby4-0-ARM

Motivation

APMSVLS-512

Testing Guidelines

Unit tests and integration tests run against the new ruby4.0 matrix entry. Layers were built and verified locally with Colima for all four supported Ruby versions × both architectures, and check_layer_size.sh passes on each (~34 MB uncompressed, well under the 100 MB cap).

Additional Notes

Dependency bumps — heads-up: these affect ALL Ruby versions, not just 4.0

The Dockerfile only defines a single global datadog gem version that every Ruby layer (3.2, 3.3, 3.4, AND 4.0) inherits. Because dd-trace-rb 2.12 cannot install on Ruby 4.0, the gem version had to advance globally:

• datadog gem 2.12 → 2.30 in the layer Dockerfile. dd-trace-rb only gained Ruby 4.0 install support in 2.24.0 (Jan 2026); 2.30 is the latest stable 2.x as of this PR. The integration test snapshot diffs show the customer-visible churn: every existing *_ruby{32,33,34}.log updates the "version":"2.12.0" → "2.30.0" line and gains the new "meta_struct":{} field that the 2.x serializer emits. This means Ruby 3.2/3.3/3.4 customers will also receive dd-trace-rb 2.30 once these layers publish — worth mentioning in the release notes.
• rake ~> 12.3 → ~> 13.0 (gemspec dev dependency). Ruby 4.0 removed ostruct from the default gems; rake 12.x requires it at load time. Rake 13.x doesn't and is compatible with 3.2/3.3/3.4.
• gemspec datadog ~> 2.12 → ~> 2.30 to match the Dockerfile pin, so fresh dev boxes on Ruby 4.0 can't resolve an unworkable 2.12.x candidate.

Cleanup folded in

• scripts/add_new_region.sh previously omitted the Datadog-Ruby3-4 and Datadog-Ruby4-0 layer names, and every -ARM variant for all Ruby versions. Both gaps are now closed (8 entries in the parallel arrays: 4 versions × amd64+arm64).
• scripts/list_layers.sh was similarly missing Datadog-Ruby3-4, Datadog-Ruby4-0, and their -ARM siblings — added.
• .gitlab/Dockerfile had a misleading # Install Ruby X.Y comment over apt-get install ruby-full. Replaced with an accurate comment explaining the version doesn't matter for this build-tooling image.

Snapshot scrubbing

Added a perl filter for aws@<version> in scripts/run_integration_tests.sh. On Ruby 4.0 the dd-trace AWS contrib reports Available?: true and emits the concrete aws-sdk-core version (e.g. aws@3.246.0) in the integrations_loaded log line, where Ruby 3.x snapshots show aws@ empty. Without the filter, the ruby40 snapshots would drift any time AWS bumps aws-sdk-core in the runtime base image. The four ruby40 snapshots are refreshed accordingly.

Types of changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog