fix(qwp): cover flushAndGetSequence's residual seal in the commit boundary

Self-review finding: flushAndGetSequence() has the same belt-and-braces
residual sealAndSwapBuffer() as close(), but the commit-boundary update
was only added to close()'s copy. A non-deferred residual publish there
left lastCommitBoundaryFsn stale, so drainOnClose targeted a lower FSN
and close() could return before the residual frame's ack -- memory-mode
data loss on JVM exit, the exact bug class this series fixes.

Author: bluestreak01

core/src/main/java/io/questdb/client/cutlass/qwp/client/QwpWebSocketSender.java

@@ -1533,6 +1533,13 @@ public long flushAndGetSequence() {
         }
         if (activeBuffer != null && activeBuffer.hasData()) {
             sealAndSwapBuffer();
+            if (!deferCommit) {
+                // Same residual-seal boundary update as close(): a
+                // non-deferred residual publish is commit-bearing and must be
+                // covered by close-time drains, or drainOnClose would return
+                // before its ack and memory-mode data could be lost on exit.
+                lastCommitBoundaryFsn = cursorEngine.publishedFsn();
+            }
         }
         cursorSendLoop.checkError();
         checkConnectionError();