fix(cache): skip disk writes for app 404 responses

[bobu-putheeckal]

Closes #73101

Summary

• Keep app page and app route 404 cache entries in memory but skip writing those generated 404 responses to disk.
• Add unit coverage for the cache writer and a production ISR regression test that confirms notFound() responses do not create static artifacts.

Testing

• PATH=./node_modules/.bin:/Users/c543982/.npm/_npx/179a5fd3f63fede5/node_modules/.bin:$PATH ./node_modules/.bin/jest --runInBand test/unit/incremental-cache/file-system-cache.test.ts
• PATH=./node_modules/.bin:/Users/c543982/.npm/_npx/179a5fd3f63fede5/node_modules/.bin:$PATH scripts/run-jest.sh --mode=start --bundler=webpack --headless -- test/production/app-dir/isr-not-found-static-files/isr-not-found-static-files.test.ts
• git diff --check
• ./node_modules/.bin/prettier --check packages/next/src/server/lib/incremental-cache/file-system-cache.ts test/unit/incremental-cache/file-system-cache.test.ts test/production/app-dir/isr-not-found-static-files/isr-not-found-static-files.test.ts 'test/production/app-dir/isr-not-found-static-files/app/isr-not-found/[slug]/page.tsx' test/production/app-dir/isr-not-found-static-files/app/layout.tsx