Summary

• Keeps zero-token hosts in topology while excluding zero-token and in-flight hosts from query plans.
• Cleans up failed host additions and fences host-add generations so stale futures cannot remove newer pools or mark a newer add down.
• Preserves successful add state when listener callbacks fail.

Review fixes

1. Aborted add future could remove a newer successful pool.
2. Stale add pool construction failure could signal host down after abort.

Tests

• uv run python -c "import sys, pytest; sys.exit(pytest.main(['-q', 'tests/unit/test_cluster.py', 'tests/unit/test_policies.py', 'tests/unit/test_control_connection.py']))"
• repo-ci fast passed: c9d52c1a1810f8331ffb7e24e512e71a79afb2ede0615cde518f801fa5f297e0