feat(scan): [4/N] address followups for reading and applying V3 deletion vectors

[mbutrovich]

Which issue does this PR close?

• Closes Validate the construction of FileScanTaskDeleteFile. #3135.

What changes are included in this PR?

Review follow-ups from #3035, per @CTTY's feedback.

• FileScanTaskDeleteFile::build() now validates deletion vectors. A deletion vector is a PositionDeletes entry stored as Puffin, and must carry referenced_data_file, content_offset, content_size_in_bytes and record_count, with non-negative Puffin coordinates. This follows FileScanTask, which already validates in build() via build_method(into = Result<FileScanTask>).
• deletion_vector_coordinates() is the single source of truth for those fields: validate() calls it at build time and CachingDeleteFileLoader at read time, so the loader's validate_deletion_vector_task is removed rather than duplicated.
• DeleteFileIndex no longer checks content_offset / content_size_in_bytes; those now surface when a matched entry is converted into a scan task. It still requires referenced_data_file, the key it indexes deletion vectors by, which cannot be deferred. From<&DeleteFileContext> becomes TryFrom, and get_deletes_for_data_file propagates the error.
• Also addresses a naming nit not tracked in Validate the construction of FileScanTaskDeleteFile. #3135 (review comment): pos_deletes_by_path is renamed to pos_deletes_by_referenced_data_file, for consistency with dvs_by_referenced_data_file.

Breaking change: FileScanTaskDeleteFile::builder().build() now returns Result<FileScanTaskDeleteFile>. This is inherent to validating in build(), and matches FileScanTask.

Deletion vector read behavior is otherwise unchanged, as is position and equality delete handling.

Are these changes tested?

Yes.

• Unit tests in task.rs for each field the builder now rejects a deletion vector for, that a valid deletion vector builds and reports its coordinates, and that a Parquet position delete file is not held to those requirements. The first seven replace the test_validate_deletion_vector_task_* tests in caching_delete_file_loader.rs, which covered the same cases against the removed helper; the cardinality tests there are unchanged, since that check still belongs to the loader.
• test_deletion_vector_missing_coordinates_is_rejected in delete_file_index.rs now asserts through get_deletes_for_data_file, where the check fires after the move.

AI Disclosure

Developed with the help of Claude Code, but I understand and support these changes.

[mbutrovich]

Just noticed #3145, I'll take a look at it and see which one is closer.

[mbutrovich]

Setting to draft. I will review #3145 and leave this as a reference until that merges.