bump minor version to 3.29.0

[zarirhamza]

What does this PR do?

Release bump from v3.28.0 to v3.29.0. This release packages up two changes merged since v3.28.0:

• APPSEC-61865 Add AppSec setup for AWS Lambda ([APPSEC-61865] Add AppSec setup for AWS Lambda #144) — adds the AppSec request/response normalization + gateway push path for API Gateway events, instruments aws_lambda AppSec, and pins dd-trace-rb to 2.34 (which ships the matching Contrib::AwsLambda watcher / WAFAddresses consumer).
• fix: compile FFI from source using the AWS Lambda image (Compile FFI from source using AWS Lambda image #153) — switches the layer build from ruby:X.Y (Debian) to public.ecr.aws/lambda/ruby:X.Y so native extensions compile against Lambda's actual glibc / libffi, then reinstalls ffi from source. Fixes the cold-start LoadError: cannot load such file -- ffi_c crash on Ruby 3.2 when DD_APPSEC_ENABLED=true.

Motivation

Cuts a release so the AppSec-enabled, FFI-fixed Datadog-Ruby3-2 / Datadog-Ruby3-3 / Datadog-Ruby3-4 / Datadog-Ruby4-0 (and -ARM variants) layers get published. This unblocks the (ruby32, appsec_tracer) scenarios currently xfailed in serverless-e2e-tests — see validation PR #226.

Testing Guidelines

Per the Lambda Layer Ruby runbook: once this PR's gitlab pipeline finishes its build layer / check layer size / unit test / integration test jobs, the publish layer sandbox jobs are manual.

Plan:

1. Wait for build + integration tests to pass.
2. Tag v3.29.0, push tag, wait for the e2e tests to pass.
3. Validate the AppSec + FFI fix end-to-end via serverless-e2e-tests#226 (un-xfails ruby32 + appsec_tracer).
4. Click the prod signing + publish jobs per runbook; publish to GovCloud; publish the gem.

Additional Notes

Diff is a single-line minor bump of MINOR = 28 → MINOR = 29 in lib/datadog/lambda/version.rb. Branch is based on current main (898e93e), so the tag cut from this release will include both #144 and #153.

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