🐛 Coerce workgraph node attributes to JSON-safe values on save

[elinscott]

Problem

save_workgraph_data assigns the serialized workgraph straight onto node attributes (workgraph_data, workgraph_data_short, workgraph_error_handlers). Node attributes must be JSON-serializable, but workgraph data can carry arbitrary Python objects that are not - the reproducing case is an Enum default picked up from a task function signature:

class SpinChannel(str, Enum):
    UP = "up"
    DOWN = "down"

@task
def screen(channel: SpinChannel = SpinChannel.UP) -> float:
    ...

Storing the process node then fails deep inside the storage backend with an unhelpful serialization error, far from the offending task definition.

Change

New _ensure_json_safe helper applied to the three attribute payloads in save_workgraph_data. It walks the structure recursively and:

1. passes through None / bool / int / float / str and containers of them unchanged;
2. keeps anything that already survives json.dumps as-is;
3. unwraps value-carrying objects (obj.value, non-callable - covers Enum members and similar wrappers) and recurses;
4. falls back to str(value) as a last resort (a lossy-but-stored representation beats an engine crash at store time)

Ordinary workgraphs (already JSON-safe) pass through untouched.

Notes

• The coercion applies only to the node-attribute copies used for display and provenance browsing; execution-side serialization of task inputs/outputs is unchanged.
• this bug possibly points to a deeper structural issue, which this PR patches superficially rather han addressing