refactor: Split graph validation and parsing from _graph.py

Move graph validation and parsing logic from `_graph.py` to dedicated utility modules (`_graph_validation.py` and `_graph_parser.py`) under `workflow/utils/`. This simplifies the core `Graph` and `Edge` model definitions.

Co-authored-by: Shangjie Chen <deanchen@google.com>
PiperOrigin-RevId: 941176423

Author: DeanChensj