Replace the existing HTTP client with a custom-configured client that skips TLS verification for favicon fetching,
and add explicit Content-Type validation to ensure only valid image responses are accepted.
This fixes potential issues with SSL certificate validation and prevents processing of non-image responses.
The icon fetching logic now checks if the target service is healthy before
attempting to fetch icons. If the health monitor reports an unhealthy status,
the function returns HTTP 503 Service Unavailable instead of proceeding.
Additionally, the icon cache lookup now includes infinite retry logic with a
15-second backoff interval, improving resilience during transient service
outages. Previously, failed lookups would not be retried.
The `route` interface was extended with a `HealthMonitor()` method to support
the health check functionality.
