fix(ci): install open-feature/sdk for Feature flags tests job

test_featureflags fails on CI with "Class OpenFeature\interfaces\provider\Reason
not found" and downstream "Class DDTrace\OpenFeature\DataDogProvider not
found" because the SDK is never installed. The bridge autoload of
_generated_openfeature.php declares DataDogProvider extends AbstractProvider;
without the SDK on the autoloader path, the parent lookup fails and the whole
bridge file aborts mid-load.

- Add tests/OpenFeature/composer.json requiring open-feature/sdk ^2.1 so the
  existing tests/%/composer.lock-php pattern rule installs it into
  tests/OpenFeature/vendor/.
- Wire test_featureflags to depend on
  tests/OpenFeature/composer.lock-php$(PHP_MAJOR_MINOR) so the install runs
  before phpunit is invoked.

The bootstrap_common walkup autoloader picks up tests/OpenFeature/vendor/
when loading OpenFeature test files, registering the SDK autoloader before
the extension autoloads the adapter bridge.

Author: leoromanovsky

Makefile

@@ -1315,7 +1315,7 @@ test_distributed_tracing_coverage:
 test_metrics: global_test_run_dependencies
 	$(call run_tests,--testsuite=metrics $(TESTS))
 
-test_featureflags: global_test_run_dependencies
+test_featureflags: global_test_run_dependencies tests/OpenFeature/composer.lock-php$(PHP_MAJOR_MINOR)
 	$(call run_tests,--testsuite=featureflags $(TESTS))
 
 benchmarks_run_dependencies: global_test_run_dependencies tests/Frameworks/Symfony/Version_5_2/composer.lock-php$(PHP_MAJOR_MINOR) tests/Frameworks/Laravel/Version_10_x/composer.lock-php$(PHP_MAJOR_MINOR) tests/Benchmarks/composer.lock-php$(PHP_MAJOR_MINOR)

tests/OpenFeature/composer.json

@@ -0,0 +1,7 @@
+{
+    "name": "datadog/dd-trace-tests-openfeature",
+    "require": {
+        "open-feature/sdk": "^2.1"
+    },
+    "minimum-stability": "stable"
+}