fix: snapshot was producing empty summary

[imor]

Which issue does this PR close?

• Closes #. There was no open issue for this. I noticed this while trying out iceberg-rust.

What changes are included in this PR?

The first line in the method SnapshotProducer::write_added_manifest (let added_data_files = std::mem::take(&mut self.added_data_files);) sets the self.added_data_files to an empty vec. SnapshotProducer::write_added_manifest is called via the call chain SnapshotProducer::commit -> SnapshotProducer::manifest_file -> SnapshotProducer::write_added_manifest). Hence, if SnapshotProducer::summary is called after calling SnapshotProducer::manifest_file, the summary produced was empty due to empty self.added_data_files.

This PR rearranges the code in SnapshotProducer::commit to make sure the produced summary is not empty.

Are these changes tested?

No new tests have been added for this as there were no previous tests.

[liurenjie1024]

Thanks @imor for this fix!