[Spark] Release the exported Arrow batch if writeBatch throws before native takes it #11

VortexDataWriter hands each batch to VortexWriter.writeBatch inside a try-with-resources ArrowArray. If writeBatch throws before the native side takes ownership, every buffer of that batch leaks.

Why

• ArrowArray.close() only frees the struct and never calls release(), so the buffers the export retained are never released.
• Two paths throw before native code takes ownership:
  • Preconditions.checkState(!closed.get(), "writer already closed") in VortexWriter.writeBatch;
  • writer_ptr <= 0 in native writeBatch.
• Once native code is entered there's no leak: FFI_ArrowArray::from_raw takes ownership straight away.

Impact

Latent: no current caller can trigger it.

• VortexDataWriter only flushes while vortexWriter is non-null, and nulls it as soon as the writer is finished or closed.
• The builder rejects a zero native pointer.

A future caller that writes after close(), or reorders cleanup, would leak the batch.

Where

• flushBatch on develop. PR #7 keeps the same pattern.

Fix

Release the array if writeBatch throws before native code takes it: call arrowArray.release() in a catch around writeBatch, or check the writer's state before exporting.