fix(auth): remove unnecessary async from sync interface methods

fix(auth): remove unnecessary async from sync interface methods

isRemoteAvailable() was declared as boolean | Promise<boolean> in the
OCPPAuthAdapter interface while both implementations return boolean
synchronously. This forced unnecessary await, Promise.resolve() wrappers,
and async propagation through the call chain.

Fix the interface to boolean and cascade through:
- RemoteAuthStrategy: getStats(), testConnectivity(), checkRemoteAvailability()
  become synchronous
- AuthStrategy.getStats(): JsonObject | Promise<JsonObject> becomes JsonObject
- OCPPAuthService.getStats(): Promise<AuthStats> becomes AuthStats
- OCPPAuthServiceImpl.getStats() becomes synchronous

Update mocks and tests to match sync signatures.