ServerAuthenticationController no longer rereads the keychain on every auth call

Resolved auth tokens are now memoized inside ServerAuthenticationController, bounded by the token’s own expiry and using the existing shared CachedValueStore for single-flight. On macOS, the keychain read serialized under concurrency and dominated per-op latency on hosted runners (packet-traced ~340ms/op CAS transfers whose wire time is ~1ms). Every call now performs one underlying read instead of one per op, with the rest of the auth path unchanged.