fix: set finish before notifying load in LanceArrowWriter setFinished method (#92)

Recently I was testing the integration between Lance and Spark.

I found that the Lance writer has a certain probability of hanging.
After some troubleshooting, I discovered this is related to the
LanceArrowWriter.setFinished method.

The original code appears to have a bug where it sets the finished
status after notifying loadNextBatch, which could cause loadNextBatch to
hang.

**Root Cause**

The ideal flow should be:
1. (thread 1) loadToken.release
2. (thread 1) finished = true
3. (thread 2) loadNextBatch
4. (thread 2) finished is true and count is 0 so return false

However, there's a chance it becomes:
1. (thread 1) loadToken.release
2. (thread 2) loadNextBatch
3. (thread 2) finished is false so return true and waiting
4. (thread 1) finished = false

If the second scenario occurs, thread 2 will hang indefinitely and
cannot receive new notifications. jstack will show stacks hanging in
LanceDataWriter.commit.

**Reproduction**

This issue is hard to reproduce. I encountered it in a very low-resource
environment (Spark executor with only 1 core 4g) when creating a new
table and writing 600 rows of data at once, where one column is a
1024-dimensional vector column.

It also occurs intermittently.

**Further Confirmation**

Although the current fix seems reasonable, I hope to get confirmation
from maintainers to avoid introducing new unknown issues.

Any comments about this. @jackye1995

Author: ColdL

lance-spark-base_2.12/src/main/java/com/lancedb/lance/spark/write/LanceArrowWriter.java

@@ -34,7 +34,7 @@ public class LanceArrowWriter extends ArrowReader {
   private final int batchSize;
 
   @GuardedBy("monitor")
-  private volatile boolean finished;
+  private volatile boolean finished = false;
 
   private final AtomicLong totalBytesRead = new AtomicLong();
   private com.lancedb.lance.spark.arrow.LanceArrowWriter arrowWriter = null;
@@ -70,8 +70,8 @@ void write(InternalRow row) {
   }
 
   void setFinished() {
-    loadToken.release();
     finished = true;
+    loadToken.release();
   }
 
   @Override

lance-spark-base_2.12/src/test/java/com/lancedb/lance/spark/write/LanceArrowWriterTest.java

@@ -26,7 +26,6 @@
 import org.apache.spark.sql.types.DataTypes;
 import org.apache.spark.sql.types.StructField;
 import org.apache.spark.sql.types.StructType;
-import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 
 import java.util.Collections;
@@ -35,8 +34,6 @@
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
-// TODO: Flaky in CI, needs to fix
-@Disabled
 public class LanceArrowWriterTest {
   @Test
   public void test() throws Exception {