Client routes: mixed CLIENT_ROUTES_CHANGE events can drop cached proxy routes

[dkropachev]

_ClientRoutesHandler.handle_client_routes_change() can remove valid cached proxy routes when a CLIENT_ROUTES_CHANGE event mixes configured and unrelated connection_id values.

Problem

The handler converts all event host_ids to host_uuids, then filters (connection_id, host_id) pairs down to configured connection_ids before querying system.client_routes.

However, it currently calls _RouteStore.merge(..., affected_host_ids=set(host_uuids)), so merge() treats every host in the original event as affected, including hosts whose event entry belonged only to an unrelated connection_id.

Because merge() removes existing routes for every affected host before adding queried rows, a mixed event can delete an existing valid route for a host that was not actually updated for this driver configuration.

A related route-state issue exists in ClientRoutesEndPoint: endpoint equality and hashing ignore original_port, so two endpoints for the same host_id and original address but different ports collapse to the same metadata key.

Expected behavior

• Partial route merges should only mark hosts from configured (connection_id, host_id) event pairs as affected.
• ClientRoutesEndPoint identity should include original_port.

Notes

This is distinct from #813: that issue is about preserving same-host connection-id stickiness when partial events omit the currently selected route. This issue is about deleting routes for hosts whose event entries were filtered out as unrelated.