Add tests for QueryPlan round-robin retry progression and exhaustion

[m-szymon]

Round-robin node selection within a single QueryPlan is not directly covered for retry progression and exhaustion.

Include:

• a failed first node within one request leads the retry to a different node, not the failed one
• repeated next_node() calls within one plan return each node at most once and None once exhausted

Expected outcome:

• retry walks the same per-request round-robin QueryPlan (via used_nodes) instead of reusing the failed node
• single-plan exhaustion returns None after all nodes are consumed

Part of #41.