Mark the test-only DDSpanContext back-compat ctor @VisibleForTesting

Signals intent: this delegating overload has no production caller (CoreTracer
uses the full ctor); its ~20 callers are tests/jmh.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dougqh

dd-trace-core/src/main/java/datadog/trace/core/DDSpanContext.java

@@ -22,6 +22,7 @@
 import datadog.trace.api.gateway.RequestContext;
 import datadog.trace.api.gateway.RequestContextSlot;
 import datadog.trace.api.internal.TraceSegment;
+import datadog.trace.api.internal.VisibleForTesting;
 import datadog.trace.api.sampling.PrioritySampling;
 import datadog.trace.api.sampling.SamplingMechanism;
 import datadog.trace.bootstrap.instrumentation.api.AgentSpanContext;
@@ -308,6 +309,7 @@ public DDSpanContext(
   }
 
   /** Back-compat ctor (no read-through parent); delegates with a null parent. */
+  @VisibleForTesting
   public DDSpanContext(
       final DDTraceId traceId,
       final long spanId,