[Java] VectorUnloader has multiple memory leak issues

從 VectorUnloader 類別開始，重點檢查 getRecordBatch() 和 appendNodes()，尤其是 issue 中描述的 retain()、compress() 以及遞迴子節點處理路徑。驗證每個位置發生失敗時都會釋放所有保留的 ArrowBuf，包括先前反覆執行中的緩衝區，並確認正常的 record-batch 序列化仍然運作。

Describe the bug, including details regarding any error messages, version, and platform.

Problem Description

The VectorUnloader class has serious memory leak issues when serializing Vector data. Any exception thrown during the process will cause previously retained ArrowBuf instances to never be released.
Root Cause
ArrowBuf uses reference counting for memory management. After calling retain(), a corresponding release() must be called to free the buffer. The code has exception-safety defects in multiple locations:

Bug 1: OOM during compression causes leak

Location: appendNodes() method, around line 75
buf.getReferenceManager().retain();
buffers.add(codec.compress(vector.getAllocator(), buf)); // OOM here causes leak
Problem: If codec.compress() throws OutOfMemoryError, retain() has already been called but buffers.add() never executes, making the ArrowBuf unreleasable.

Bug 2: Compression failure in loop causes partial leak

Location: appendNodes() method, around lines 78-85
for (ArrowBuf buf : fieldBuffers) {
    buf.getReferenceManager().retain();
    buffers.add(codec.compress(vector.getAllocator(), buf)); // Any failure leaks all previously retained buffers
}
Problem: Any compression failure in the loop causes all previously retained buffers to leak.

Bug 3: Exception during recursive child vector processing causes leak

Location: appendNodes() method, around lines 87-89
for (FieldVector child : vector.getChildrenFromFields()) {
    appendNodes(child, nodes, buffers, variadicBufferCounts); // Exception here leaks buffers from current level
}
Problem: An exception during recursive child processing prevents release of buffers already retained at the current level.

Bug 4: Iteration failure in top-level loop causes cumulative leaks

Location: getRecordBatch() method, around lines 53-58
for (FieldVector vector : root.getFieldVectors()) {
    appendNodes(vector, nodes, buffers, variadicBufferCounts); // Any failure leaks all previously retained buffers
}
Problem: Each iteration retains buffers, and any failure causes cumulative leaks of all previously retained buffers. This is an accumulated leak.

Impact

• All code using VectorUnloader.getRecordBatch()
• Leak probability increases significantly when processing large datasets or when compressor runs low on memory
• Leaked ArrowBuf instances persist until process termination
  Suggested Fix

1. Use try-finally to ensure release() is called even when exceptions occur
2. Or add buffers to the list before calling retain() and compress()
3. Use transactional error handling for the entire getRecordBatch() operation, rolling back all buffers on failure