[CLEAN] Synthetic Benchmark PR #30531 - feat: Add conversation variable persistence layer

[bar-qodo]

Benchmark PR langgenius#30531

Type: Clean (correct implementation)

Original PR Title: feat: Add conversation variable persistence layer
Original PR Description: > [!IMPORTANT]

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Summary

• Fixes [Refactor/Chore] Add graph-layer conversation variable persistence handling langgenius/dify#30550
• Add a graph-layer conversation variable persistence handler that consumes Variable Assigner outputs and writes updates via ConversationVariableUpdater.
• Keep persistence concerns out of nodes by handling updates centrally when node-success events are processed.
• Dependencies: none.

Screenshots

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Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

Original PR URL: langgenius#30531