Faster CLI auth and CAS cache transfers by removing per-operation keychain reads on macOS

Faster CAS artifact uploads and downloads on macOS. The CLI now memoizes the resolved auth token in ServerAuthenticationController so repeated authenticationToken() calls no longer trigger a keychain read via securityd on every operation. Previously, the CAS compilation-cache daemon re-read the macOS keychain on each artifact request, serializing under concurrency and adding roughly 340ms per op on hosted macOS runners even though the actual network transfer took about 1ms. The cached resolution is bounded by the token’s own expiry, uses single-flight so concurrent requests perform one read, and benefits every CLI caller that needs the auth token, not only the CAS daemon.