Validate the construction of `FileScanTaskDeleteFile`.

We should validate this when building `FileScanTaskDeleteFile`.
```
isDv == FileScanTaskDeleteFile.content_type == PosDel && FileScanTaskDeleteFile.file_format == puffin
fn build() {
if (isDv) { validate_deletion_vector_task() }
}

```

_Originally posted by @CTTY in https://github.com/apache/iceberg-rust/pull/3035#discussion_r3919914422_

Start at the FileScanTaskDeleteFile construction and build() entry point, then trace how isDv, content_type, file_format, PosDel, and puffin are represented. Determine where validate_deletion_vector_task() should be invoked and add coverage for the stated condition. Done means invalid deletion-vector task combinations are rejected during construction.

Written by the indexing model from the issue text.