Cached flags collide between two client instances on different environments (cache not namespaced per environment)

[adamvialpando]

When two Flagsmith client instances run in the same app on different environment keys with caching enabled, they can serve or overwrite each other's cached flags.

Use case: a host app uses the Flagsmith iOS SDK, and a third-party SDK embedded in the same app also uses the Flagsmith SDK, each configured with its own environment key against the same base URL.

Observed behaviour: with caching on, flags from one environment can be returned for the other, or the two instances overwrite each other's cached values.

Likely cause (to validate): the cache appears keyed on request URL only. The environment is sent as the X-Environment-Key header and the flags path is fixed, so two instances on the same base URL share a cache entry. On iOS this is compounded by Flagsmith.shared being a singleton.

Steps to reproduce:

1. Create two Flagsmith clients in one app, each with a different environment key, same API base URL, caching enabled.
2. Fetch flags on client A, then client B.
3. Observe client B serving client A's cached flags, or the two overwriting each other.

Proposed direction: namespace the cache per environment key so instances don't collide