[Test] Fix assertion for number of message chunks (elastic#122571) (elastic#122717)

Depending on the position of the split and the size of the extra chunk,
the test may send out one more chunk than the assertion expects. This PR
ensures the assertoin is more accurate to count for this scenario.

Resolves: elastic#122349

Author: ywangd

modules/transport-netty4/src/test/java/org/elasticsearch/transport/netty4/Netty4WriteThrottlingHandlerTests.java

@@ -109,6 +109,7 @@ public void testThrottleLargeCompositeMessage() {
             Netty4WriteThrottlingHandler.MAX_BYTES_PER_WRITE * fullSizeChunks + extraChunkSize
         );
         int splitOffset = randomIntBetween(0, messageBytes.length);
+        int lastChunkSizeOfTheFirstSplit = splitOffset % Netty4WriteThrottlingHandler.MAX_BYTES_PER_WRITE;
         final BytesReference message = CompositeBytesReference.of(
             new BytesArray(messageBytes, 0, splitOffset),
             new BytesArray(messageBytes, splitOffset, messageBytes.length - splitOffset)
@@ -120,7 +121,9 @@ public void testThrottleLargeCompositeMessage() {
         assertFalse(promise.isDone());
         embeddedChannel.flush();
         assertTrue(promise.isDone());
-        assertThat(seen, hasSize(oneOf(fullSizeChunks, fullSizeChunks + 1)));
+        // If the extra chunk size is greater than the last chunk size for the first half of the split, it means we will need to send
+        // (extraChunkSize - lastChunkSizeOfTheFirstSplit) bytes as the very last chunk of the entire message.
+        assertThat(seen, hasSize(oneOf(fullSizeChunks, fullSizeChunks + 1 + (extraChunkSize > lastChunkSizeOfTheFirstSplit ? 1 : 0))));
         assertTrue(capturingHandler.didWriteAfterThrottled);
         assertBufferEquals(Unpooled.compositeBuffer().addComponents(true, seen), message);
     }