feat: expose k_top_nodes in DynamicRouteProviderBuilder

[shilingwang]

Background

LatencyRoutingSnapshot has long supported a k_top_nodes option that limits routing to only the top k nodes ranked by latency score. However, this option became inaccessible after #694 removed the generic snapshot parameter from DynamicRouteProviderBuilder, leaving set_k_top_nodes effectively dead code.

Changes

Added k_top_nodes: Option<usize> as an explicit parameter to both DynamicRouteProviderBuilder::new() and DynamicRouteProviderBuilder::from_components(). Pass Some(k) to cap routing to the top k nodes by latency score, or None to use all healthy nodes (existing default behaviour).

Removed #[allow(unused)] from LatencyRoutingSnapshot::set_k_top_nodes, since it is now called from the builder.
Updated all internal call sites to pass None.

Usage

// Limit routing to the 5 lowest-latency nodes
DynamicRouteProviderBuilder::new(seeds, client, Some(5)).build()
// Use all healthy nodes (unchanged default)
DynamicRouteProviderBuilder::new(seeds, client, None).build()