chore: compress large plugin_model obj in redis

[hjlarry]

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Summary

• Compress large plugin_model_providers Redis payloads before caching.
• Keep small payloads uncompressed with a 64KB threshold to avoid unnecessary CPU cost.
• Add a versioned prefix for compressed values and keep backward compatibility with existing plain JSON cache entries.

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	52.27%	52.27%	+0.00%
Strict coverage	51.78%	51.78%	+0.00%
Typed symbols	31,977	31,979	+2
Untyped symbols	29,478	29,478	0
Modules	2969	2969	0