Summary

• merge the redundant nested eviction checks in PluginCacheService so unload only runs when the cache entry both expired or was removed and is a plugin loader, matching existing patterns in the infrastructure layer
• add an XML summary on RegisterEvictionCallback to document why the callback exists and keep inline docs consistent with CONTRIBUTING guidance

Testing

• dotnet test FlowSynx.sln (fails: installed SDK is 8.0.415 while the solution targets net9.0)

Fixes #703