fix: persist nested subgraph widget values on save

[vivekchavan14]

Fixes #4930 where nested subgraph input values were not being persisted when saving workflows. The issue occurred because SubgraphNode wasn't enabling widget serialization when it had promoted widgets.

Changes:

• Enable serialize_widgets when widgets are promoted to SubgraphNode
• Ensure serialization is enabled during configuration phase
• Clean up serialization flag when widgets are removed
• Add comprehensive test coverage for nested subgraph scenarios

This ensures widget values in nested subgraphs persist across save/load cycles, resolving the user-reported issue where values would revert to defaults after reopening workflows.

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