Add EVP flagevaluation system tests

[leoromanovsky]

Motivation

Server-side FFE EVP flagevaluation needs system-test coverage for the worker-visible event contract and SDK-side aggregation behavior before language-by-language rollout.

Changes

• Adds tests/ffe/test_flag_eval_evp.py for basic EVP emission, repeated-count aggregation, bounded context, runtime default, load aggregation, bounded burst aggregation, high-cardinality aggregation, and production-cap degradation.
• Adds batched /ffe request support to weblog fixtures so degradation is exercised with 10,050 evaluations without making the test an HTTP throughput benchmark.
• Adds feature_flags_evp_flagevaluation as a distinct feature marker from dynamic evaluation, exposures, and OTel evaluation metrics.
• Wires manifests so SDK languages keep this new test disabled for first review with missing_feature (FFL-2446); non-applicable proxy/lambda/OTel manifests are marked irrelevant.

Decisions

• Tests validate visible EVP fields from /api/v2/flagevaluations without vendoring dd-source schemas.
• OpenFeature reason is neither accepted in payloads nor used as an aggregation dimension.
• targeting_rule is expected only when real targeting-rule metadata exists; current no-rule fixtures assert it is omitted.
• Degradation is reached naturally by one request with 10,050 distinct evaluations for one flag; degraded events must omit context and targeting key while preserving visible flag, variant, allocation, and count fields.
• System-tests do not force private SDK queue caps. The bounded burst and degradation tests exercise async handoff, aggregation, and degradation end-to-end; SDK-internal queue-overflow accounting remains SDK unit-test territory.
• Language enablement happens in separate draft sibling PRs after each SDK has local passing evidence.

Evidence

Base PR checks on current head 2e7ad616bc525ee76698f34a686da549701b1914:

./run.sh TEST_THE_TEST tests/test_the_test/test_conventions.py::test_utils

Result: 1 passed in 0.74s.

Current manifest state:

• golang, nodejs, python, ruby, java, .NET, php: tests/ffe/test_flag_eval_evp.py: missing_feature (FFL-2446).
• Proxy, lambda, and OTel-only manifests are marked irrelevant where the server-side EVP weblog scenario does not apply.

Validated sibling evidence includes PHP #7160: ./run.sh FEATURE_FLAGGING_AND_EXPERIMENTATION --library php --weblog apache-mod-8.2 -k "test_flag_eval_evp" - PASS, 8 passed, 2577 deselected in 63.53s. Degradation payload from that run: events=10001, total=10050, full_total=10000, degraded_count=1, degraded_total=50.

[github-actions]

CODEOWNERS have been resolved as:

tests/ffe/test_flag_eval_evp.py                                         @DataDog/feature-flagging-and-experimentation-sdk @DataDog/system-tests-core
tests/ffe/utils/__init__.py                                             @DataDog/feature-flagging-and-experimentation-sdk @DataDog/system-tests-core
tests/ffe/utils/fixtures.py                                             @DataDog/feature-flagging-and-experimentation-sdk @DataDog/system-tests-core
manifests/cpp.yml                                                       @DataDog/dd-trace-cpp
manifests/cpp_httpd.yml                                                 @DataDog/dd-trace-cpp
manifests/cpp_kong.yml                                                  @DataDog/system-tests-core
manifests/cpp_nginx.yml                                                 @DataDog/dd-trace-cpp
manifests/dotnet.yml                                                    @DataDog/apm-dotnet @DataDog/asm-dotnet
manifests/envoy.yml                                                     @DataDog/system-tests-core
manifests/golang.yml                                                    @DataDog/dd-trace-go-guild
manifests/haproxy.yml                                                   @DataDog/system-tests-core
manifests/java.yml                                                      @DataDog/asm-java @DataDog/apm-java
manifests/java_lambda.yml                                               @DataDog/system-tests-core
manifests/java_otel.yml                                                 @DataDog/system-tests-core
manifests/nodejs.yml                                                    @DataDog/dd-trace-js
manifests/nodejs_lambda.yml                                             @DataDog/system-tests-core
manifests/nodejs_otel.yml                                               @DataDog/system-tests-core
manifests/php.yml                                                       @DataDog/apm-php @DataDog/asm-php
manifests/python.yml                                                    @DataDog/apm-python @DataDog/asm-python
manifests/python_lambda.yml                                             @DataDog/apm-python @DataDog/asm-python
manifests/python_otel.yml                                               @DataDog/apm-python @DataDog/asm-python
manifests/ruby.yml                                                      @DataDog/ruby-guild @DataDog/asm-ruby
manifests/ruby_lambda.yml                                               @DataDog/system-tests-core
manifests/rust.yml                                                      @DataDog/apm-rust
tests/ffe/README.md                                                     @DataDog/feature-flagging-and-experimentation-sdk @DataDog/system-tests-core
tests/ffe/test_exposures.py                                             @DataDog/feature-flagging-and-experimentation-sdk @DataDog/system-tests-core
tests/ffe/test_flag_eval_metrics.py                                     @DataDog/feature-flagging-and-experimentation-sdk @DataDog/system-tests-core
utils/_features.py                                                      @DataDog/system-tests-core
utils/build/docker/dotnet/weblog/Controllers/FeatureFlagEvaluatorController.cs  @DataDog/apm-dotnet @DataDog/asm-dotnet @DataDog/system-tests-core
utils/build/docker/golang/app/_shared/common/ffe.go                     @DataDog/dd-trace-go-guild @DataDog/system-tests-core
utils/build/docker/java/spring-boot/src/main/java/com/datadoghq/system_tests/springboot/featureflag/FeatureFlagEvaluatorController.java  @DataDog/apm-java @DataDog/asm-java @DataDog/system-tests-core
utils/build/docker/nodejs/express/app.js                                @DataDog/dd-trace-js @DataDog/system-tests-core
utils/build/docker/php/common/ffe.php                                   @DataDog/apm-php @DataDog/system-tests-core
utils/build/docker/python/flask/app.py                                  @DataDog/apm-python @DataDog/asm-python @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

[datadog-prod-us1-4]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

_{This comment will be updated automatically if new data arrives.
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