Add topology-change tests for host-id identity and endpoint replacement

[dkropachev]

Parent epic: #921

Problem

The host identity and endpoint replacement refactor changes behavior across metadata, control connection refresh, sessions, pools, policies, and route state. The current tests do not provide enough focused coverage for host-id identity, endpoint movement, replacement semantics, and stale async publication.

Desired coverage

Add focused unit and integration coverage for:

• Host.__eq__ and Host.__hash__ using stable identity;
• same host_id, changed endpoint;
• same endpoint, different host_id;
• metadata _hosts and _host_id_by_endpoint consistency;
• control connection refresh resolving hosts by host_id first;
• session pool renewal/removal after endpoint replacement;
• load-balancing policy host membership after endpoint movement;
• token-aware routing and replica filtering under stable host identity;
• stale pool creation and stale connection callbacks being discarded;
• client route stickiness/state when keyed by stable host identity.

Scope

• Prefer unit tests for identity semantics, metadata index behavior, and generation fencing.
• Add integration tests only where live cluster behavior is required, such as real endpoint/IP movement or replacement.
• Keep long-running integration cases gated by the existing test environment conventions.

Acceptance criteria

• New unit tests fail on endpoint-based Host identity and pass with host-id identity.
• Tests cover both endpoint movement and host replacement/rebirth.
• Stale async publication tests cover the bugs from ResponseFuture can return or borrow connections from a stale endpoint pool #857, HostConnection can signal failures from stale or replaced pools #858, HostConnection can publish replacement shard connections after endpoint changes #859, and Session pool creation can publish stale pools to replacement hosts #860.
• Integration coverage is added or explicitly deferred with a documented reason.

Part of #921. Supports #867, #922, #923, #924, #925, and #813.