fix: Fix typo in Artifact.Builder#extensions

Author: fjuma

spec/src/main/java/io/a2a/spec/Artifact.java

@@ -75,7 +75,7 @@ public Builder metadata(Map<String, Object> metadata) {
         }
 
         public Builder extensions(List<String> extensions) {
-            this.extensions = this.extensions = (extensions == null) ? null : List.copyOf(extensions);
+            this.extensions = (extensions == null) ? null : List.copyOf(extensions);
             return this;
         }