Performance: Review static caching and cache invalidation in the helper

Problem/Motivation

The helper injects cache.default and keeps internal static caches. Cache keys must include the language code and the current user's permissions, otherwise multilingual sites and authenticated users can be served another context's data. There is also no documented invalidation strategy.

Proposed resolution

• Audit every cache key and include langcode plus any relevant context.
• Store cache items with entity cache tags so edits invalidate them automatically.
• Consider a dedicated cache.myrest bin so the data can be cleared independently.
• Add kernel tests proving that content edits invalidate cached structures.

Remaining tasks

Audit the caching code and add tags plus tests.

User interface changes

None.

API changes

None.