feat: dev snippet backend

[FFXN]

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Summary

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 make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods

[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	46.00%	46.00%	0.00%
Strict coverage	45.52%	45.52%	0.00%
Typed symbols	24,791	24,791	0
Untyped symbols	29,410	29,410	0
Modules	2756	2756	0