fix(workflow): dump validated schemas in JSON mode

[a2105z]

Summary

• Use model_dump(mode="json") when converting validated node schema results to dicts so when_used="json" serializers run and values like Decimal / datetime / non-str Enum stay JSON-serializable
• Align BaseNode._to_serializable, validate_node_data, validate_schema, and resume-response dumping with the documented Event.output contract
• Add regression coverage for the reported Decimal case plus datetime/Enum

Fixes #6747

Test plan

• uv run pytest tests/unittests/utils/test_schema_utils.py tests/unittests/workflow/utils/test_rehydration_utils.py -q (95 passed)
• Repro from output_schema validation dumps in pydantic python mode, so the node result is not JSON-serializable #6747: before fix json.dumps raised TypeError on Decimal; after fix returns {"price": {"amount": 29.99}}