cluster: fence host state and pool races by endpoint

[dkropachev]

Summary

• Add identity-keyed event fence state for host liveness and session pool creation so concurrent up/down handling, queued-up replay, reconnectors, and pool callbacks can be invalidated by epoch instead of mutable Host equality.
• Replay queued UP events after active DOWN handling finishes, while preserving newer down events, removals, and endpoint swaps that should cancel stale replay.
• Scope status events, reconnection success, connection failures, auth-failure quarantine, and pool cleanup to the endpoint observed when the work started, including SNI and client-routes endpoints whose identity is not just address/port.
• Make session pool creation and removal race-safe: reuse in-flight pool creation, discard stale creations after endpoint changes, remove pools by host identity when endpoint hashes change, and avoid signaling down from stale keyspace/auth failures.
• Carry expected endpoints through control-connection status changes, defunct connection returns, shard-aware replacement connections, and scheduler de-duplication so delayed work cannot affect a replacement endpoint.

Driver surface and risks

• Touches internal cluster host-state handling, session pool lifecycle, control-connection status handling, scheduler uniqueness, and HostConnection reconnect/shard-aware paths.
• No CQL protocol or public API change is intended. The main compatibility risk is changed timing around concurrent host up/down and endpoint migration handling; the branch adds focused unit coverage for those race cases.

Fixes #317

Test coverage

• Added unit coverage in tests/unit/test_cluster.py for pool-creation races, stale up/down callbacks, queued-up replay, endpoint swaps, reconnector fencing, and non-retryable auth failure handling.
• Added unit coverage in tests/unit/test_control_connection.py for preserving endpoint identity in delayed status events.
• Added unit coverage in tests/unit/test_host_connection_pool.py for ignoring defunct stale connections after endpoint/client-route changes.

Validation

• uv run pytest tests/unit/test_cluster.py -q
• git diff --check

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.