Fix caught exception type in rollback test

flushSync() wraps the NullPointerException from sendBinary()
on a null client in a LineSenderException via its
catch (Throwable) block. The test was catching the unwrapped
NullPointerException, which never arrived, causing the test
to fail with an unhandled LineSenderException.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mtopolnik

core/src/test/java/io/questdb/client/test/cutlass/qwp/client/QwpDeltaDictRollbackTest.java

@@ -24,6 +24,7 @@
 
 package io.questdb.client.test.cutlass.qwp.client;
 
+import io.questdb.client.cutlass.line.LineSenderException;
 import io.questdb.client.cutlass.qwp.client.InFlightWindow;
 import io.questdb.client.cutlass.qwp.client.QwpWebSocketSender;
 import io.questdb.client.test.AbstractTest;
@@ -65,9 +66,9 @@ public void testSyncFlushFailureDoesNotAdvanceMaxSentSymbolId() throws Exception
                 // because client is null (we never actually connected)
                 try {
                     sender.flush();
-                    Assert.fail("Expected NullPointerException from null client");
-                } catch (NullPointerException expected) {
-                    // sendBinary() on null client
+                    Assert.fail("Expected LineSenderException from null client");
+                } catch (LineSenderException expected) {
+                    // sendBinary() on null client, wrapped by flushSync()
                 }
 
                 // The fix: maxSentSymbolId must remain -1 because the send failed.