[Improvement] Move snapshot ID capture in expireFiles() to after delete commit (#4169)

Move snapshot ID capture in expireFiles() to after delete commit

The snapshot ID passed to RewriteFiles.validateFromSnapshot() was
captured before DeleteFiles.commit(), making it stale for the
subsequent RewriteFiles operation. Although the current validation
path is benign (replacedDataFiles is empty so conflict detection is
skipped), passing a pre-commit snapshot ID is semantically incorrect
and fragile against future Iceberg validation changes.

Move the capture to after delete.commit() so that
validateFromSnapshot() always receives the up-to-date snapshot.

Signed-off-by: Jiwon Park <jpark92@outlook.kr>

Author: j1wonpark

amoro-format-iceberg/src/main/java/org/apache/amoro/formats/iceberg/maintainer/IcebergTableMaintainer.java

@@ -875,7 +875,6 @@ public static Expression getDataExpression(
   }
 
   public void expireFiles(ExpireFiles expiredFiles, long expireTimestamp) {
-    long snapshotId = IcebergTableUtil.getSnapshotId(table, false);
     Queue<DataFile> dataFiles = expiredFiles.dataFiles;
     Queue<DeleteFile> deleteFiles = expiredFiles.deleteFiles;
     if (dataFiles.isEmpty() && deleteFiles.isEmpty()) {
@@ -890,6 +889,7 @@ public void expireFiles(ExpireFiles expiredFiles, long expireTimestamp) {
     delete.commit();
     // expire delete files
     if (!deleteFiles.isEmpty()) {
+      long snapshotId = IcebergTableUtil.getSnapshotId(table, false);
       RewriteFiles rewriteFiles = table.newRewrite().validateFromSnapshot(snapshotId);
       deleteFiles.forEach(rewriteFiles::deleteFile);
       rewriteFiles.set(