Fix evaluation reason logic: SPLIT precedence (ADR-004), DEFAULT for catch-all allocs, date window precludes STATIC (ADR-003)

Author: typotter

products/feature-flagging/feature-flagging-api/src/main/java/datadog/trace/api/openfeature/DDEvaluator.java

@@ -386,10 +386,7 @@ private static <T> ProviderEvaluation<T> resolveVariant(
     final ProviderEvaluation<T> result =
         ProviderEvaluation.<T>builder()
             .value(mappedValue)
-            .reason(
-                !isEmpty(allocation.rules)
-                    ? Reason.TARGETING_MATCH.name()
-                    : !isEmpty(split.shards) ? Reason.SPLIT.name() : Reason.STATIC.name())
+            .reason(resolveReason(allocation, split, flag))
             .variant(variant.key)
             .flagMetadata(metadataBuilder.build())
             .build();
@@ -400,6 +397,25 @@ private static <T> ProviderEvaluation<T> resolveVariant(
     return result;
   }
 
+  private static String resolveReason(
+      final Allocation allocation, final Split split, final Flag flag) {
+    // ADR-004: SPLIT overrides TARGETING_MATCH when both rules and shard contributed
+    if (!isEmpty(allocation.rules) && !isEmpty(split.shards)) {
+      return Reason.SPLIT.name();
+    }
+    if (!isEmpty(allocation.rules)) {
+      return Reason.TARGETING_MATCH.name();
+    }
+    if (!isEmpty(split.shards)) {
+      return Reason.SPLIT.name();
+    }
+    // No rules, no shards (vacuous split). STATIC only when this is the sole allocation
+    // with no date-window constraints (ADR-003: time-gated result is not permanently stable).
+    final boolean hasDateWindow = allocation.startAt != null || allocation.endAt != null;
+    final boolean isSoleStaticAlloc = flag.allocations.size() == 1 && !hasDateWindow;
+    return isSoleStaticAlloc ? Reason.STATIC.name() : Reason.DEFAULT.name();
+  }
+
   private static Object resolveAttribute(final String name, final EvaluationContext context) {
     // Special handling for "id" attribute: if not explicitly provided, use targeting key
     if ("id".equals(name) && !context.keySet().contains(name)) {