Add test case for OF.7: empty targeting key is valid

OF.7 specifies that empty string shall be accepted as a targeting key
and evaluation should proceed as normal.

Changes:
- Added test case in DDEvaluatorTest for empty targeting key
- Modified TestCase utility to use a custom EvaluationContext wrapper
  that preserves empty targeting keys (OpenFeature SDK's MutableContext
  converts empty string to null, which breaks OF.7 compliance)

Author: leoromanovsky

products/feature-flagging/api/src/test/java/datadog/trace/api/openfeature/DDEvaluatorTest.java

@@ -213,6 +213,11 @@ private static List<TestCase<?>> evaluateTestCases() {
         new TestCase<>("default")
             .flag("simple-string")
             .result(new Result<>("default").reason(ERROR.name()).errorCode(TARGETING_KEY_MISSING)),
+        // OF.7: Empty string is a valid targeting key - evaluation should proceed as normal
+        new TestCase<>("default")
+            .flag("simple-string")
+            .targetingKey("")
+            .result(new Result<>("test-value").reason(TARGETING_MATCH.name()).variant("on")),
         new TestCase<>("default")
             .flag("non-existent-flag")
             .targetingKey("user-123")

products/feature-flagging/api/src/test/java/datadog/trace/api/openfeature/util/TestCase.java

@@ -1,21 +1,53 @@
 package datadog.trace.api.openfeature.util;
 
 import dev.openfeature.sdk.ErrorCode;
+import dev.openfeature.sdk.EvaluationContext;
 import dev.openfeature.sdk.MutableContext;
 import dev.openfeature.sdk.Structure;
 import dev.openfeature.sdk.Value;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Set;
 
 public class TestCase<E> {
 
   public Class<E> type;
   public String flag;
   public E defaultValue;
-  public final MutableContext context = new MutableContext();
+  private final MutableContext baseContext = new MutableContext();
+  private String targetingKeyOverride;
+  private boolean hasTargetingKeyOverride = false;
   public Result<E> result;
 
+  // Custom context wrapper that preserves empty targeting key (OF.7 compliance)
+  public final EvaluationContext context = new EvaluationContext() {
+    @Override
+    public String getTargetingKey() {
+      return hasTargetingKeyOverride ? targetingKeyOverride : baseContext.getTargetingKey();
+    }
+
+    @Override
+    public Value getValue(String key) {
+      return baseContext.getValue(key);
+    }
+
+    @Override
+    public Set<String> keySet() {
+      return baseContext.keySet();
+    }
+
+    @Override
+    public Map<String, Value> asMap() {
+      return baseContext.asMap();
+    }
+
+    @Override
+    public Map<String, Object> asObjectMap() {
+      return baseContext.asObjectMap();
+    }
+  };
+
   @SuppressWarnings("unchecked")
   public TestCase(final E defaultValue) {
     this.type = (Class<E>) defaultValue.getClass();
@@ -28,37 +60,39 @@ public TestCase<E> flag(String flag) {
   }
 
   public TestCase<E> targetingKey(final String targetingKey) {
-    context.setTargetingKey(targetingKey);
+    // Preserve the targeting key directly to support empty string (OF.7)
+    this.targetingKeyOverride = targetingKey;
+    this.hasTargetingKeyOverride = true;
     return this;
   }
 
   public TestCase<E> context(final String key, final String value) {
-    context.add(key, value);
+    baseContext.add(key, value);
     return this;
   }
 
   public TestCase<E> context(final String key, final Integer value) {
-    context.add(key, value);
+    baseContext.add(key, value);
     return this;
   }
 
   public TestCase<E> context(final String key, final Double value) {
-    context.add(key, value);
+    baseContext.add(key, value);
     return this;
   }
 
   public TestCase<E> context(final String key, final Boolean value) {
-    context.add(key, value);
+    baseContext.add(key, value);
     return this;
   }
 
   public TestCase<E> context(final String key, final Structure value) {
-    context.add(key, value);
+    baseContext.add(key, value);
     return this;
   }
 
   public TestCase<E> context(final String key, final List<Value> value) {
-    context.add(key, value);
+    baseContext.add(key, value);
     return this;
   }