Add tests for affinity routing under retry and scope fallback/recovery

[m-szymon]

Affinity routing is only tested for deterministic single-node targeting; its behavior under retries and scope fallback/recovery is not covered.

Include:

• affinity + retry within one request (retry walks the same per-request affinity QueryPlan, not the failed node)
• affinity + scope fallback when the preferred scope becomes unavailable
• affinity + recovery back to the primary scope once it returns

Expected outcome:

• a failed affinity-selected node leads to the next node from the same plan, not a restart or reuse
• routing stays deterministic within the live fallback set during failure
• routing returns to the primary scope after recovery

Part of #41.