Fix mutation of cache objects causing base64 responses

[m-sanders]

Redis handler mutated the original cacheHandlerValue object Next.js uses for responses. When parseBuffersToStrings() converted Buffers to base64, Next.js sent the mutated object to users. We shallow clone value object before mutation to isolate changes from Next.js's reference.

How was this tested?

Manually

$ npm run build && npm run start
$ redis-cli -p 6379 -n 1 -e flushdb
$ curl http://localhost:3000/manifest.webmanifest
{"name":"Next.js App","short_name":"Next.js App","description":"Next.js App","start_url":"/","display":"standalone","background_color":"#fff","theme_color":"#fff","icons":[{"src":"/favicon.ico","sizes":"any","type":"image/x-icon"}]}%

[m-sanders]

@AyronK this is also a bug in Next.js 15.x. Can we get this released please?