Replace endpoint-based host lookup with host-id-first topology updates

[dkropachev]

Parent epic: #921

Problem

Several topology paths still use endpoint/address as a way to find or identify an existing host. That conflicts with the target model where host_id is the only stable host identity and endpoint is connectivity metadata.

Known risky areas include:

• control connection refresh logic that searches by endpoint before falling back to host_id;
• Cluster.add_host() behavior that short-circuits on endpoint mappings;
• metadata secondary indexes such as _host_id_by_endpoint being treated as identity rather than lookup state;
• session pools, load-balancing policies, token maps, and route state that may rely on Host equality/hash semantics.

Desired behavior

• When host_id is known, topology updates resolve hosts by host_id only.
• Endpoint lookup is retained only as a secondary discovery/event-translation index.
• Same endpoint with a different host_id is treated as a replacement/rebirth, not the same host.
• Same host_id with a different endpoint is passed to the explicit endpoint replacement flow tracked by Introduce explicit endpoint replacement flow for existing host_id #923.
• Host.__eq__ and Host.__hash__ no longer depend on endpoint; see Host equality/hash depend on mutable endpoint #867.

Scope

• Audit host lookup/update callsites in cassandra/cluster.py, cassandra/metadata.py, cassandra/pool.py, cassandra/policies.py, client routes, and tests.
• Remove endpoint-first host identity decisions when host_id is available.
• Keep _host_id_by_endpoint as an index, but make its ownership and update points explicit.
• Update tests that currently expect endpoint-based Host equality or set/dict behavior.

Acceptance criteria

• Existing host lookup during topology refresh is host-id-first.
• No internal session/pool/policy path depends on endpoint-based Host equality for identity.
• Endpoint lookup remains available for address-based lookup APIs and event translation.
• Tests cover same endpoint/different host_id and same host_id/different endpoint detection.

Part of #921. Related to #867 and #382.