Add Ruby EVP fork isolation system test

[leoromanovsky]

Motivation

Ruby EVP flagevaluation has fork/prefork behavior that is hard to prove from unit tests alone: the child process must not flush parent-process queued EVP evaluation state. This PR pre-wires Rails72 system-test coverage for that runtime path without activating it yet.

Changes

• Add a Rails72 /ffe/fork-isolation endpoint that evaluates one flag in the parent process, evaluates another in a forked child, and flushes OpenFeature exposure and EVP flagevaluation writers deterministically.
• Add a Ruby-specific EVP flagevaluation system test asserting the child-emitted batch does not contain the parent flag.
• Keep the shared Ruby EVP flagevaluation manifest disabled as missing_feature (FFL-2446).
• Keep the new Ruby fork-isolation test disabled for Rails72 until DataDog/dd-trace-rb#5896 lands.
• Pin Rails72 openfeature-sdk to >= 0.5.1 so provider hooks are available when Ruby EVP coverage is activated.

Decisions

• Do not activate Ruby EVP system-tests in this PR. The Ruby tracer implementation is still in DataDog/dd-trace-rb#5896, so merging activation into system-tests first would advertise coverage before the default Ruby tracer can satisfy it.
• Keep the fork/prefork regression proof in system-tests because it validates real Rails + OpenFeature + tracer + Agent EVP dataflow across process boundaries. Ruby repo unit tests still cover the writer-level fork reset behavior, but they do not prove the full weblog/Agent payload path.
• Avoid asserting that the parent never emits its own event; the harness legitimately flushes the parent process later. The test only rejects parent state appearing in the child flush batch.

Validation

Validation was run locally against DataDog/dd-trace-rb#5896 by building Rails72 with that local Ruby tracer source:

• ./build.sh --library ruby --weblog-variant rails72 --images weblog --binary-path /tmp/system-tests-ruby-binary
• Verified image installed datadog 2.37.0.dev, openfeature-sdk 0.5.1, libdatadog 36.0.0.1.0.
• TEST_LIBRARY=ruby ./run.sh FEATURE_FLAGGING_AND_EXPERIMENTATION tests/ffe/test_flag_eval_evp_ruby.py::Test_FFE_EVP_Flagevaluation_Ruby_Fork_Isolation --library ruby -> 1 passed in 24.65s
  - TEST_LIBRARY=ruby ./run.sh FEATURE_FLAGGING_AND_EXPERIMENTATION tests/ffe/test_flag_eval_evp.py tests/ffe/test_flag_eval_evp_ruby.py --library ruby -> 8 passed, 1 skipped in 46.39s
• Ruby syntax checks for Rails72 controllers/routes.
• python3 -m compileall -q tests/ffe/test_flag_eval_evp_ruby.py
• venv/bin/ruff check tests/ffe/test_flag_eval_evp_ruby.py
• targeted mypy for tests/ffe/test_flag_eval_evp_ruby.py and tests/ffe/test_flag_eval_evp.py
• venv/bin/python utils/manifest/validate.py
• venv/bin/yamllint -s manifests/ruby.yml
• git diff --check

[github-actions]

CODEOWNERS have been resolved as:

tests/ffe/test_flag_eval_evp_ruby.py                                    @DataDog/feature-flagging-and-experimentation-sdk @DataDog/system-tests-core
manifests/ruby.yml                                                      @DataDog/ruby-guild @DataDog/asm-ruby
utils/build/docker/ruby/rails72/Gemfile                                 @DataDog/ruby-guild @DataDog/asm-ruby @DataDog/system-tests-core
utils/build/docker/ruby/rails72/Gemfile.lock                            @DataDog/ruby-guild @DataDog/asm-ruby @DataDog/system-tests-core
utils/build/docker/ruby/rails72/app/controllers/internal_controller.rb  @DataDog/ruby-guild @DataDog/asm-ruby @DataDog/system-tests-core
utils/build/docker/ruby/rails72/app/controllers/open_feature_controller.rb  @DataDog/ruby-guild @DataDog/asm-ruby @DataDog/system-tests-core
utils/build/docker/ruby/rails72/config/routes.rb                        @DataDog/ruby-guild @DataDog/asm-ruby @DataDog/system-tests-core

[datadog-prod-us1-5]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

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