fix: avoid jest namespace in fetch test helper

[TechWithTy]

Summary

• replace the direct usage of the Jest namespace in the shared fetch test helper with a narrow mock interface
• centralize access to the mocked fetch instance to avoid relying on globally-available Jest types during production builds
• move @types/jest into runtime dependencies so the type definitions are present during production installs

Testing

• pnpm run build (fails: Next.js cannot download external Google Fonts in this environment)

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https://chatgpt.com/codex/tasks/task_e_68ead36d5b8c8329acadeb96acda7e72

[vercel]

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Project	Deployment	Preview	Comments	Updated (UTC)
deal-scale	Error			Oct 11, 2025 10:40pm