isHealthy

Checks if the indexer service is healthy and reachable.

Performs a lightweight health check by calling the root endpoint and verifying the service returns a successful status.

This method does not throw exceptions - it returns false for any error condition (network failure, timeout, unhealthy response).

Note: Uses Throwable instead of Exception because on Kotlin/JS, Ktor network failures throw JavaScript Error objects that map to Throwable, not Exception.