fix(transaction): move flag assignment after auto-commit

Move last_update_committed_ assignment to after auto-commit attempt.

If Commit() fails, ICEBERG_RETURN_UNEXPECTED returns early and the flag
remains false, preventing the transaction from incorrectly reporting the
update as committed when it actually failed.

Author: shangxinli

src/iceberg/transaction.cc

@@ -65,12 +65,12 @@ Status Transaction::Apply(std::vector<std::unique_ptr<TableUpdate>> updates) {
     update->ApplyTo(*metadata_builder_);
   }
 
-  last_update_committed_ = true;
-
   if (auto_commit_) {
-    ICEBERG_RETURN_UNEXPECTED(Commit());
+    auto result = Commit();
+    ICEBERG_RETURN_UNEXPECTED(result);
   }
 
+  last_update_committed_ = true;
   return {};
 }