Add topology-generation fencing for pools and connections

[dkropachev]

Parent epic: #921

Problem

Pool and connection work can run asynchronously while topology changes are happening. A pool or connection operation started for an older endpoint or host state can publish results after the host has moved, been replaced, or had its current pool renewed.

This underlies several existing bugs:

• ResponseFuture can return or borrow connections from a stale endpoint pool #857: ResponseFuture can borrow from or return to stale endpoint pools.
• HostConnection can signal failures from stale or replaced pools #858: stale HostConnection failures can affect replacement/current pools.
• HostConnection can publish replacement shard connections after endpoint changes #859: replacement shard connections can be published after endpoint changes.
• Session pool creation can publish stale pools to replacement hosts #860: async session pool creation can publish stale pools to replacement hosts.

Desired behavior

Pools and connection operations should carry enough topology identity to prove they still belong to current state before publishing effects.

Likely shape:

• introduce a topology generation/version per host/session pool lifecycle;
• stamp pool creation, connection replacement, and async callbacks with the generation they started under;
• validate generation before publishing pools, connections, failure signals, or stream-id returns;
• close/discard stale work instead of attaching it to current state.

Scope

• Define generation ownership: metadata host generation, session pool generation, or a combined token.
• Add checks around pool publication, connection replacement, failure signaling, and request completion paths.
• Integrate with endpoint replacement from Introduce explicit endpoint replacement flow for existing host_id #923.
• Keep the mechanism small enough to land before larger pool-manager work from Fix pool management #382.

Acceptance criteria

• Stale async pool creation cannot overwrite the current pool.
• Stale connection replacement cannot publish into the current pool.
• Failure signals from retired pools cannot mark replacement/current host state down.
• ResponseFuture returns stream ids and borrows connections only from the pool it actually used or from a verified-current pool.
• Regression tests cover the races tracked by 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.

Part of #921. Related to #382.