fix(server): Prefer ready Kura cache endpoints

Resolves the cache endpoint selection issue where accounts with ready Kura servers still needed client opt-in.

Prefer ready account Kura cache endpoints in the default cache endpoint resolution path. The readiness signal is the existing Kura endpoint mirror in account_cache_endpoints, which is only populated after the Kura server endpoint is available. If an account has no ready Kura endpoint, the previous custom endpoint and default endpoint fallback behavior is preserved.

This also removes the cache endpoint feature-flag selector, so Xcode cache clients move to Kura automatically once the account has a ready Kura server.

How to test locally

• mix format --check-formatted lib/tuist/accounts.ex lib/tuist_web/controllers/api/cache_controller.ex test/tuist/accounts_test.exs test/tuist_web/controllers/api/cache_controller_test.exs
• mix test test/tuist/accounts_test.exs:4042 test/tuist_web/controllers/api/cache_controller_test.exs:139 test/tuist_web/controllers/api/cache_controller_test.exs:192 test/tuist_web/controllers/api/cache_controller_test.exs:221 test/tuist_web/controllers/api/cache_controller_test.exs:256