Enable Ruby OpenFeature evaluation on older weblogs

[leoromanovsky]

Motivation

APM needs Ruby Feature Flags validation beyond the newest Rails weblog. The oldest Ruby system-tests image that can use the current useful OpenFeature SDK line is Ruby 3.1, so this broadens non-metric flag evaluation coverage to that floor and documents the practical support boundary for customers using OpenFeature hooks.

Changes

• Adds OpenFeature SDK support and /ffe evaluation endpoints to the sinatra41 weblog.
• Enables tests/ffe/test_dynamic_evaluation.py for uds-sinatra and direct sinatra41 on Ruby 3.1 while keeping rails72 coverage.
• Leaves exposure, EVP, and OTel metric test declarations unchanged.

Decisions

• Use openfeature-sdk ~> 0.5.1 instead of a broader ~> 0.5 range so Ruby 3.1 does not resolve the Ruby 3.4-only 0.6.x line.
• Treat Ruby 3.1 as the oldest practical OpenFeature system-test target because the current useful OpenFeature Ruby SDK line requires Ruby >= 3.1.
• Keep OTel metrics scoped to rails72; this PR proves core flag evaluation behavior on older Ruby, not hook-heavy metrics validation.

Validation Evidence

System Tests

• Ruby implementation PR: feat(open_feature): emit server-side EVP flagevaluation dd-trace-rb#5896
• Ruby EVP flagevaluation system-tests PR: Enable EVP flagevaluation system tests for Ruby #7184
• Ruby OpenFeature runtime documentation PR: Document Ruby OpenFeature runtime recommendation documentation#37674
• This PR enables the Ruby 3.1 system-test surface for non-metric OpenFeature evaluation coverage.

[github-actions]

CODEOWNERS have been resolved as:

manifests/ruby.yml                                                      @DataDog/ruby-guild @DataDog/asm-ruby
utils/build/docker/ruby/sinatra41/Gemfile                               @DataDog/ruby-guild @DataDog/asm-ruby @DataDog/system-tests-core
utils/build/docker/ruby/sinatra41/Gemfile.lock                          @DataDog/ruby-guild @DataDog/asm-ruby @DataDog/system-tests-core
utils/build/docker/ruby/sinatra41/app.rb                                @DataDog/ruby-guild @DataDog/asm-ruby @DataDog/system-tests-core