fix: preserve variable splitting result details

[zhonggang666]

Summary

• Preserve the full variable splitting result map for execution details.
• Keep existing runtime references working when an output field is named result.
• Restore previous variable outputs from the saved result map when continuing a workflow from a form node.

Root cause

Variable splitting returned {'result': response, **response}. If a split output field was also named result, it overwrote the internal result map with a scalar string. Continuing a later form then restored the node and called .items() on that string.

Validation

• python -m py_compile apps\application\flow\step_node\variable_splitting_node\impl\base_variable_splitting_node.py
• git diff --check
• Local equivalent dictionary-collision check for a split field named result

Fixes #6377