Fix LWT routing: preserve Paxos leader order in TokenAwarePolicy

[mykaul]

Summary

LWT (Lightweight Transaction) queries rely on Paxos consensus, where the first natural replica in the token ring acts as the Paxos leader. Routing LWT queries directly to the Paxos leader avoids an extra network hop and reduces Paxos round-trips from 4 to 3, significantly improving latency.

TokenAwarePolicy.make_query_plan() currently passes all replicas through yield_in_order(), which re-sorts them by distance (LOCAL_RACK → LOCAL → REMOTE). This is correct for regular queries, but breaks Paxos leader routing for LWT queries — the leader may be demoted if it's in a different rack than the client (with RackAwareRoundRobinPolicy).

Additionally, the tablet code path constructs replicas from the child policy's round-robin order (child.make_query_plan()), which completely loses the natural token-ring order for LWT queries regardless of child policy.

This is modeled after gocql's pickLWTReplicas() which yields replicas in natural order without distance reordering for LWT queries.

Changes

When query.is_lwt() returns True:

1. Tablet path: Resolve replicas from tablet.replicas in natural order using get_host_by_host_id() instead of filtering through the child policy's round-robin output
2. Non-tablet path: Yield replicas in their natural token-ring order (from get_replicas()), skipping only down/IGNORED hosts — do NOT pass through yield_in_order() distance bucketing
3. Non-replica fallback hosts still use distance-based ordering (unchanged)
4. Non-LWT queries: Behavior is completely unchanged

Related Issues

• Fixes LWT routing: RackAwareRoundRobinPolicy demotes Paxos leader #780 (RackAwareRoundRobinPolicy demotes Paxos leader)
• Fixes LWT routing: Tablet path loses natural token-ring order (Paxos leader not prioritized) #781 (Tablet path loses ring order for LWT)

Tests

Added LWTTokenAwareRoutingTest class with 11 new tests covering:

• Basic LWT preserves ring order (Paxos leader first)
• Non-LWT still uses distance-based ordering
• LWT with RackAwareRoundRobinPolicy preserves leader even when in different rack
• LWT skips down hosts
• LWT skips IGNORED hosts (remote DC)
• LWT with tablet routing preserves natural order
• Non-LWT tablet routing preserves round-robin (child policy) ordering
• LWT with shuffle disabled still preserves order
• Non-LWT with shuffle enabled randomizes replicas
• Non-LWT with DCAwareRoundRobinPolicy preserves behavior
• Non-LWT with RackAwareRoundRobinPolicy preserves rack-aware ordering

All 93 tests in tests/unit/test_policies.py pass.

Note

This fix is against master. A follow-up will apply the same logical fix on top of PR #651 (query plan optimization), which has the same bugs in its refactored code structure.