Deprecate + @VisibleForTesting all three test-only DDSpanContext shims

The three delegating overloads exist only to spare tests the param churn from
signature changes; none has a production caller. Mark them all @deprecated
(steer new code to the full ctor) and @VisibleForTesting (signal intent).

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

Author: dougqh

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

@@ -206,6 +206,8 @@ public class DDSpanContext
    */
   private volatile Map<String, Object> metaStruct = EMPTY_META_STRUCT;
 
+  @Deprecated
+  @VisibleForTesting
   public DDSpanContext(
       final DDTraceId traceId,
       final long spanId,
@@ -256,6 +258,8 @@ public DDSpanContext(
         null);
   }
 
+  @Deprecated
+  @VisibleForTesting
   public DDSpanContext(
       final DDTraceId traceId,
       final long spanId,
@@ -309,6 +313,7 @@ public DDSpanContext(
   }
 
   /** Back-compat ctor (no read-through parent); delegates with a null parent. */
+  @Deprecated
   @VisibleForTesting
   public DDSpanContext(
       final DDTraceId traceId,