GH-1116: [Java] Fix compressed buffer prefix write and ZSTD dstCapacity (#1119)

## What

Two fixes in the compression codec:

1. **`AbstractCompressionCodec.compress()`: capture `writerIndex()`
once**

The previous code read `uncompressedBuffer.writerIndex()` at multiple
sites — for the size comparison and again after `doCompress()` to
populate the 8-byte uncompressed-length prefix. Capture the value once
at the top of `compress()` and reuse it for the empty-buffer check, the
size comparison, and the prefix, so all three consumers see the same
value.

2. **`ZstdCompressionCodec.doCompress()`: `dstCapacity` overstated by 8
bytes**

`Zstd.compressUnsafe(dst, dstSize, ...)` expects `dstSize` to be the
available space from `dst`. The code offsets `dst` by 8 bytes past the
prefix but passed `8 + maxSize` instead of `maxSize`. The
`compressBound()` headroom hides this in practice, but the parameter was
semantically wrong. Pass `maxSize`.

## Tests

Covered by the existing round-trip tests (`testEmptyBuffer`,
`testReadWriteStream`, `testReadWriteFile`, etc.). I was not able to
construct a minimal reproducer for the original `declaredUncompressed=0`
symptom on the unfixed code, so both fixes are conservative correctness
improvements derived from code inspection rather than failing-then-green
regression tests.

Author: LuciferYang

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

@@ -44,7 +44,7 @@ protected ArrowBuf doCompress(BufferAllocator allocator, ArrowBuf uncompressedBu
     long bytesWritten =
         Zstd.compressUnsafe(
             compressedBuffer.memoryAddress() + CompressionUtil.SIZE_OF_UNCOMPRESSED_LENGTH,
-            dstSize,
+            maxSize,
             /*src*/ uncompressedBuffer.memoryAddress(),
             /* srcSize= */ uncompressedBuffer.writerIndex(),
             /* level= */ this.compressionLevel);

vector/src/main/java/org/apache/arrow/vector/compression/AbstractCompressionCodec.java

@@ -29,7 +29,11 @@ public abstract class AbstractCompressionCodec implements CompressionCodec {
 
   @Override
   public ArrowBuf compress(BufferAllocator allocator, ArrowBuf uncompressedBuffer) {
-    if (uncompressedBuffer.writerIndex() == 0L) {
+    // GH-1116: capture writerIndex() once so the empty-buffer check, size
+    // comparison, and uncompressed-length prefix all see the same value.
+    long uncompressedLength = uncompressedBuffer.writerIndex();
+
+    if (uncompressedLength == 0L) {
       // shortcut for empty buffer
       ArrowBuf compressedBuffer = allocator.buffer(CompressionUtil.SIZE_OF_UNCOMPRESSED_LENGTH);
       compressedBuffer.setLong(0, 0);
@@ -41,7 +45,6 @@ public ArrowBuf compress(BufferAllocator allocator, ArrowBuf uncompressedBuffer)
     ArrowBuf compressedBuffer = doCompress(allocator, uncompressedBuffer);
     long compressedLength =
         compressedBuffer.writerIndex() - CompressionUtil.SIZE_OF_UNCOMPRESSED_LENGTH;
-    long uncompressedLength = uncompressedBuffer.writerIndex();
 
     if (compressedLength > uncompressedLength) {
       // compressed buffer is larger, send the raw buffer