[BUG]: `FileMetaData.c_obj` can be null and segfault on dereference

`FileMetaData` has no `__cinit__` that default-constructs `c_obj`, so any instance created via
`FileMetaData.__new__(FileMetaData)` (bypassing `__init__`, which raises) has a null `c_obj`.
Any `dereference(self.c_obj)` call then segfaults instead of raising a Python exception:

https://github.com/NVIDIA/cudf/blob/d4ecacec0cbdcb420a786416ff3b9e50111f0ca9/python/pylibcudf/pylibcudf/io/parquet_metadata.pyx#L473-L480

Previously `c_obj` was an embedded (non-pointer) value and could not be null, so this is a
regression in safety introduced by PR #23558 (which switched `FileMetaData` to hold a
`unique_ptr` so it could be shared/reused, e.g. from `hybrid_scan.pyx`).

`Scalar` has the same `unique_ptr` + `from_libcudf`-only pattern elsewhere in pylibcudf, so this
may be worth fixing generally rather than just for `FileMetaData`.

## Related

- PR #23558 (introduced the `unique_ptr` change)
- https://github.com/NVIDIA/cudf/pull/23558#discussion_r3739313433

Start in python/pylibcudf/pylibcudf/io/parquet_metadata.pyx around lines 473-480 and review PR #23558 to understand the unique_ptr change. Check how FileMetaData instances created through __new__ behave, then compare the related Scalar pattern; done means invalid instances raise a Python exception instead of segfaulting.

Written by the indexing model from the issue text.