[CLEAN] Synthetic Benchmark PR #30563 - feat(graph-engine): add command to update variables at runtime

[bar-qodo]

Benchmark PR langgenius#30563

Type: Clean (correct implementation)

Original PR Title: feat(graph-engine): add command to update variables at runtime
Original PR Description: > [!IMPORTANT]

1. Make sure you have read our contribution guidelines
2. Ensure there is an associated issue and you have been assigned to it
3. Use the correct syntax to link this PR: Fixes #<issue number>.

Summary

Adds a new GraphEngine command to update variables at runtime, wiring it through the Redis command channel, manager API, and command handler. Includes unit tests covering command serialization and variable pool updates. Fixes langgenius#30562.

Screenshots

Before	After
N/A	N/A

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#30563