Stop HandlerExceptionTest racing the async dispatch

Both tests slept a fixed 500ms and then asserted the response had been
sent. The handler runs asynchronously, so on a loaded machine the sleep
expires first: the test failed 4 of 5 local runs at the previous commit
and twice in a row on CI.

Poll for the expected message to a 10s deadline instead. 10 consecutive
local runs pass, and the happy path is faster than the old fixed sleep.

Author: markpollack

acp-core/src/test/java/com/agentclientprotocol/sdk/client/HandlerExceptionTest.java

@@ -3,6 +3,7 @@
  */
 package com.agentclientprotocol.sdk.client;
 
+import java.time.Duration;
 import java.util.Map;
 import java.util.function.Function;
 
@@ -23,7 +24,7 @@ class HandlerExceptionTest {
      * to a proper JSON-RPC error response with code -32603 (Internal Error).
      */
     @Test
-    void handlerExceptionConvertedToJsonRpcError() throws InterruptedException {
+    void handlerExceptionConvertedToJsonRpcError() {
         MockAcpClientTransport transport = new MockAcpClientTransport();
         String errorMessage = "File not found: /nonexistent.txt";
 
@@ -45,8 +46,7 @@ void handlerExceptionConvertedToJsonRpcError() throws InterruptedException {
         );
         transport.simulateIncomingMessage(request);
 
-        // Wait for async processing
-        Thread.sleep(500);
+        awaitSentMessages(transport, 1);
 
         // Verify it's a JSON-RPC error response
         assertThat(transport.getSentMessages()).hasSize(1);
@@ -66,7 +66,7 @@ void handlerExceptionConvertedToJsonRpcError() throws InterruptedException {
      * Verifies that IOException from file operations is also converted to JSON-RPC error.
      */
     @Test
-    void ioExceptionConvertedToJsonRpcError() throws InterruptedException {
+    void ioExceptionConvertedToJsonRpcError() {
         MockAcpClientTransport transport = new MockAcpClientTransport();
 
         // Create a handler that throws IOException (wrapped in RuntimeException per Java patterns)
@@ -86,8 +86,7 @@ void ioExceptionConvertedToJsonRpcError() throws InterruptedException {
         );
         transport.simulateIncomingMessage(request);
 
-        // Wait for async processing
-        Thread.sleep(500);
+        awaitSentMessages(transport, 1);
 
         assertThat(transport.getSentMessages()).hasSize(1);
         AcpSchema.JSONRPCResponse response = (AcpSchema.JSONRPCResponse) transport.getSentMessages().get(0);
@@ -97,4 +96,22 @@ void ioExceptionConvertedToJsonRpcError() throws InterruptedException {
 
         client.close();
     }
+
+    /**
+     * Waits for the transport to have sent at least {@code expected} messages. The
+     * handler runs asynchronously, so a fixed sleep races the dispatch and fails
+     * intermittently on a loaded machine; poll to a generous deadline instead.
+     */
+    private static void awaitSentMessages(MockAcpClientTransport transport, int expected) {
+        long deadline = System.nanoTime() + Duration.ofSeconds(10).toNanos();
+        while (transport.getSentMessages().size() < expected && System.nanoTime() < deadline) {
+            try {
+                Thread.sleep(10);
+            }
+            catch (InterruptedException ex) {
+                Thread.currentThread().interrupt();
+                return;
+            }
+        }
+    }
 }