Unwrap failure in testCancelRequestWhenFailingFetchingPages (#121597) (#121615)

The thrown exception may be wrapped in an ExecutionException; therefore,
we need to unwrap it before verification.

Closes #121596
Closes #118193

Author: dnhatn

x-pack/plugin/esql/src/internalClusterTest/java/org/elasticsearch/xpack/esql/action/EsqlActionTaskIT.java

@@ -432,7 +432,9 @@ protected void doRun() throws Exception {
             }
             Exception failure = expectThrows(Exception.class, () -> future.actionGet().close());
             EsqlTestUtils.assertEsqlFailure(failure);
-            assertThat(failure.getMessage(), containsString("failed to fetch pages"));
+            Throwable cause = ExceptionsHelper.unwrap(failure, IOException.class);
+            assertNotNull(cause);
+            assertThat(cause.getMessage(), containsString("failed to fetch pages"));
             // If we proceed without waiting for pages, we might cancel the main request before starting the data-node request.
             // As a result, the exchange sinks on data-nodes won't be removed until the inactive_timeout elapses, which is
             // longer than the assertBusy timeout.