Fix buffer0 leak in QwpWebSocketSender constructor

If the second MicrobatchBuffer allocation (buffer1) throws,
the already-allocated buffer0 leaks its native memory. Neither
connect() nor createForTesting() catch constructor failures,
so the cleanup must happen in the constructor itself. Wrap
the buffer1 allocation in a try-catch that closes buffer0
before re-throwing.

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

Author: mtopolnik

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

@@ -183,7 +183,12 @@ private QwpWebSocketSender(
         if (inFlightWindowSize > 1) {
             int microbatchBufferSize = Math.max(DEFAULT_MICROBATCH_BUFFER_SIZE, autoFlushBytes * 2);
             this.buffer0 = new MicrobatchBuffer(microbatchBufferSize, autoFlushRows, autoFlushBytes, autoFlushIntervalNanos);
-            this.buffer1 = new MicrobatchBuffer(microbatchBufferSize, autoFlushRows, autoFlushBytes, autoFlushIntervalNanos);
+            try {
+                this.buffer1 = new MicrobatchBuffer(microbatchBufferSize, autoFlushRows, autoFlushBytes, autoFlushIntervalNanos);
+            } catch (Throwable t) {
+                buffer0.close();
+                throw t;
+            }
             this.activeBuffer = buffer0;
         }
     }