fix(arrow): load equality deletes on fields nested in structs

[JeonDaehong]

Which issue does this PR close?

• Closes Scan fails when an equality delete uses a nested field #3450.

What changes are included in this PR?

Equality ids may name a field nested in a struct (the spec allows it and EqualityDeleteWriter writes it), but loading such a delete file failed with field not found:

• Schema evolution of the delete file only maps top-level field ids, so the delete file is now read by the top-level fields that hold the equality ids.
• The delete predicate used the leaf name (f), which fails to bind or binds to a top-level column with the same name. It now uses the full name (s.f). test_delete_file_loader_parse_equality_deletes expected the leaf name sa for a nested field, so its expected string changes to s.sa.

Applying the delete to data rows also needs predicates on nested leaves in the row filter, which #2906 adds. With #2906 merged locally, an end-to-end scan deletes the expected rows (including rows whose parent struct is null). I'll add that end-to-end test once #2906 lands.

cc @brgr-s, as discussed in #2961.

Are these changes tested?

Yes. test_nested_field_equality_delete_predicate covers loading and the full-name predicate (the table also has a top-level column with the same leaf name). 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]

@mbutrovich could you take a look when you have time? This fixes loading equality deletes on nested fields (#3450); applying them on scan also needs #2906, which you've been reviewing.

[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.