feat: Use dashmap for health monitor

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Bug: Health Checks Fail Due to DashMap Race Condition

A race condition in the health monitoring loop, introduced by the DashMap migration, can cause health checks to be silently skipped or misapplied. The loop iterates over DashMap entries to collect router IDs, then attempts to retrieve and check the corresponding monitor via get_mut() within an async future. If an entry is removed or replaced between iteration and get_mut(), the health check is silently skipped (returning Ok(())) or the wrong monitor is checked. This bypasses the concurrency control of the previous RwLock implementation, potentially leading to unmonitored failed providers.