GH-38725: [Java] decompression in Lz4CompressionCodec.java does not s…

…et writer index (#38840)

### Rationale for this change

The `doDecompress` function in `Lz4CompressionCodec` misses writing the index when it is compared with the functionality in `ZstdCompressionCodec`. This PR fixes that issue. 

### What changes are included in this PR?

Writes the index for the decompressed ArrowBuf. 

### Are these changes tested?

No

### Are there any user-facing changes?

No
* Closes: #38725

Lead-authored-by: Vibhatha Lakmal Abeykoon <vibhatha@gmail.com>
Co-authored-by: vibhatha <vibhatha@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>

java/compression/src/main/java/org/apache/arrow/compression/Lz4CompressionCodec.java

@@ -79,6 +79,7 @@ protected ArrowBuf doDecompress(BufferAllocator allocator, ArrowBuf compressedBu
     byte[] outBytes = out.toByteArray();
     ArrowBuf decompressedBuffer = allocator.buffer(outBytes.length);
     decompressedBuffer.setBytes(/*index=*/0, outBytes);
+    decompressedBuffer.writerIndex(decompressedLength);
     return decompressedBuffer;
   }
 

java/compression/src/test/java/org/apache/arrow/compression/TestCompressionCodec.java

@@ -117,6 +117,12 @@ private List<ArrowBuf> deCompressBuffers(CompressionCodec codec, List<ArrowBuf>
     return outputBuffers;
   }
 
+  private void assertWriterIndex(List<ArrowBuf> decompressedBuffers) {
+    for (ArrowBuf decompressedBuf : decompressedBuffers) {
+      assertTrue(decompressedBuf.writerIndex() > 0);
+    }
+  }
+
   @ParameterizedTest
   @MethodSource("codecs")
   void testCompressFixedWidthBuffers(int vectorLength, CompressionCodec codec) throws Exception {
@@ -139,6 +145,7 @@ void testCompressFixedWidthBuffers(int vectorLength, CompressionCodec codec) thr
     List<ArrowBuf> decompressedBuffers = deCompressBuffers(codec, compressedBuffers);
 
     assertEquals(2, decompressedBuffers.size());
+    assertWriterIndex(decompressedBuffers);
 
     // orchestrate new vector
     IntVector newVec = new IntVector("new vec", allocator);
@@ -180,6 +187,7 @@ void testCompressVariableWidthBuffers(int vectorLength, CompressionCodec codec)
     List<ArrowBuf> decompressedBuffers = deCompressBuffers(codec, compressedBuffers);
 
     assertEquals(3, decompressedBuffers.size());
+    assertWriterIndex(decompressedBuffers);
 
     // orchestrate new vector
     VarCharVector newVec = new VarCharVector("new vec", allocator);