Add missing @see entries

Author: kabir

spec/src/main/java/io/a2a/spec/A2AClientError.java

@@ -18,6 +18,8 @@
  * @see A2AClientException for general client exceptions
  * @see A2AClientHTTPError for HTTP-specific errors
  * @see A2AClientJSONError for JSON-specific errors
+ * @see A2AClientInvalidStateError for invalid state errors
+ * @see A2AClientInvalidArgsError for invalid argument errors
  */
 public class A2AClientError extends RuntimeException {
     /**