improve segment cache lru typesafety (#87129)
The LRU and cache map code was using any types in several places
`(MapEntry<any>`,` type LRUNode = MapEntry<any>`) which meant we had no
type safety for those operations. This masked a bug fixed in #87124
where `deleteFromCacheMap(tail.value)` was being called incorrectly -
the `any` typing let it compile when it shouldn't have.
This PR introduces a `LRUNode` type that captures the structure needed
for heterogeneous LRU operations (since the LRU holds both
`RouteCacheEntry` and `SegmentCacheEntry`). The generic `MapEntry<V>` is
kept for type-safe map operations, while `LRUNode` is used for deletion
and LRU logic.
