fix(scan): validate deletion vector delete tasks

[linhongyu510]

Summary

• validate deletion vector invariants when FileScanTaskDeleteFile is built
• migrate delete-file fixtures and callers to the validated builder result shape
• remove duplicate single-task deletion-vector structural checks from the loader and delete-file index

Testing

• cargo test -p iceberg scan::task::tests::test_delete_file_builder --lib
• cargo test -p iceberg --lib delete_file_loader
• cargo test -p iceberg --lib row_filter
• cargo test -p iceberg --lib caching_delete_file_loader
• cargo test -p iceberg --lib delete_file_index
• cargo test -p iceberg --lib
• cargo clippy -p iceberg --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check

Closes #3135.

[linhongyu510]

CI follow-up pushed in 8ff9d12: removed the internal design note that failed the Apache license-header check, made the helper private, and updated the validated builder public API baseline. Verified locally with 15 targeted scan tests, rustfmt, clippy -D warnings, LICENSE/NOTICE checks, and cargo-public-api 0.51.0 output comparison.

[blackmwk]

We should remove all pub fields, and add pub getter to avoid inconsistent changes.

[linhongyu510]

Addressed the review in 0228cd4: all FileScanTaskDeleteFile fields are private, with read-only getters used by internal callers; the public API baseline is updated. Verified 66 focused tests, workspace fmt, Clippy with -D warnings, public API checks, and git diff --check.