fix(toppreise): solve infinite recursion on detail page and add in-memory cache

[tazztone]

• Fixes an infinite recursion bug in processProductDetailPage when fetchSingleProductPriceStats returns negative cache data (unavailable: true), which was locking up the microtask queue. It now asserts fetchedStats has a valid tiefstpreis and is not unavailable.
• Introduces an in-memory Map cache inside getCachedPriceStats and setCachedPriceStats to prevent excessive localStorage reads and writes and improve performance.
• Bumps userscript version to 2.18.20 and updates all tests to mock memoryCache alongside localStorage.

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PR created automatically by Jules for task 7579988177813958509 started by @tazztone

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