func_to_form_spec copies non-JSON-serializable defaults (Enum, date, object) into the schema #15

Found by the adversarial review of i2mint/uf#6.

func_to_form_spec (via the function-to-schema path) copies a parameter's raw Python default into the JSON Schema default. For an Enum member, a datetime.date, or any arbitrary object, the resulting spec is not JSON-serializable. In uf this makes /api/functions/<name>/spec return 500 {"error": "Object of type Color is not JSON serializable"}.

Repro:

import enum, json
from ju.rjsf import func_to_form_spec
class Color(enum.Enum):
    red = 'red'
def f(c: Color = Color.red): ...
json.dumps(func_to_form_spec(f, nest_under_field=None))  # TypeError

Suggested: map defaults through a JSON-coercion step (Enum → .value plus enum list; date/datetime → ISO string; otherwise drop the default rather than emit something unserializable). Until i2mint/uf#6, uf users on runners without dill silently got uf's basic fallback, so this rarely surfaced through uf.