Apply read-through naming to the withParent override

Consistent with the mechanism-branch rename: name the cast local
optimizedParent rather than p (can't be `parent` — that's the method arg).

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

Author: dougqh

internal-api/src/main/java/datadog/trace/api/TagMap.java

@@ -1936,11 +1936,11 @@ public TagMap immutableCopy() {
 
   @Override
   public TagMap withParent(TagMap parent) {
-    OptimizedTagMap p = (OptimizedTagMap) parent;
-    if (p != null && !p.frozen) {
+    OptimizedTagMap optimizedParent = (OptimizedTagMap) parent;
+    if (optimizedParent != null && !optimizedParent.frozen) {
       throw new IllegalStateException("read-through parent must be frozen");
     }
-    this.parent = p;
+    this.parent = optimizedParent;
     return this;
   }