PIPE-684: handle per-MS field lists and streamline center_field_ids
- Support both string and per-MS sequence field selections in `center_field_ids`, fixing `AttributeError` from `.split()` when `inputs.field` is a list (e.g. in VLASS).
- Leverage `ms.get_fields(task_arg=...)` in `center_field_ids` to natively resolve field IDs, names, and selections via domain tools.
- Pass `fields` instead of flattened `field_names` in `phasecenter` to align with the per-MS heuristics contract.
- Add type hints and docstrings for `phasecenter` and `center_field_ids`.
