fix(scan): apply equality deletes from an all-void partition spec globally

[JeonDaehong]

Which issue does this PR close?

None, found while reading DeleteFileIndex.

What changes are included in this PR?

The spec says equality deletes stored with an unpartitioned spec are global. Java decides this with spec.isUnpartitioned(), which is also true when every field is a void transform (what a v1 table keeps after dropping a partition field). Rust checked whether the partition tuple was empty, so such deletes were indexed under a [null] partition and never matched data files from the earlier, partitioned spec. Deleted rows came back.

DeleteFileContext now carries the delete file's spec, and the index uses PartitionSpec::is_unpartitioned. If the spec is missing, it falls back to the old check. Position deletes are unchanged: Java scopes them by spec and partition too.

Are these changes tested?

Yes. test_void_spec_equality_delete_is_global fails on main (0 deletes applied) and passes here. cargo test -p iceberg --lib, clippy and fmt are clean.

AI Disclosure

I used Claude Code to find this, write the change and run the tests. I reviewed it before opening the PR.

[JeonDaehong]

@CTTY @mbutrovich could you take a look when you have time? Small fix in DeleteFileIndex: equality deletes under an all-void partition spec are global in Java but were never applied to older-spec data files here.

[JeonDaehong]

Converted to draft to stay within the new limit of 3 open PRs (#3465). The change is still ready for review; I will mark it ready again when one of my other PRs lands.