fix: Update A2AException and A2AClientError so that they extend RuntimeException instead of IOException (#379)

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Fixes #320  🦕

Author: fjuma

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

@@ -1,6 +1,9 @@
 package io.a2a.spec;
 
-public class A2AClientError extends Exception {
+/**
+ * Base exception for A2A Client errors.
+ */
+public class A2AClientError extends RuntimeException {
     public A2AClientError() {
     }
 

spec/src/main/java/io/a2a/spec/A2AException.java

@@ -1,11 +1,9 @@
 package io.a2a.spec;
 
-import java.io.IOException;
-
 /**
  * Exception to indicate a general failure related to the A2A protocol.
  */
-public class A2AException extends IOException {
+public class A2AException extends RuntimeException {
 
     /**
      * Constructs a new {@code A2AException} instance. The message is left blank ({@code null}), and no