refactor - replace inner dict[ str, bool | int | str ] in project_samples_metadata with a typed dataclass; demultiplex/core.py, _build_project_sample_metadata()

[georgemarselis-nvi]

The inner dict in project_samples_metadata currently uses a union type bool | int | str for its values. Replace with a dataclass with explicit per-field types for transfer_to_vigas, vigas_project_id, transfer_to_nird, and nird_location. Low priority - correctness is not affected.

@dataclasses.dataclass
class SampleTransferMetadata:
transfer_to_vigas: bool
vigas_project_id: int
transfer_to_nird: bool
nird_location: str