[Bug]: num_deleted_rows is 0 for partition-only DELETEs

### What happened? DELETE with a partition only predicate correctly removes files but reports `num_deleted_rows = 0` The delete works and matching files are removed when `num_removed_files` is correct, table contents are right. However, only the row count metric is wrong. This is separate from #4355 because that fix covers the scan/rewrite path for row level deletes. The partition only fast path in `delete.rs` returns early after collecting Remove actions and never populates rowc ount metrics. ### Expected behavior `num_deleted_rows` should reflect the actual rows deleted. If the count can be derived from removed file metadata/stats, return it. If it can't be derived safely, document that rather than returning a misleading 0. ### Operating System None ### Binding None ### Bindings Version _No response_ ### Steps to reproduce _No response_ ### Relevant logs ```shell ```