Update server-common/src/main/java/org/a2aproject/sdk/server/auth/AuthenticatedUser.java

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: Sh1bari

server-common/src/main/java/org/a2aproject/sdk/server/auth/AuthenticatedUser.java

@@ -1,6 +1,6 @@
 package org.a2aproject.sdk.server.auth;
 
-import jakarta.annotation.Nullable;
+import org.jspecify.annotations.Nullable;
 import org.a2aproject.sdk.util.Assert;
 
 import java.util.Map;