Close WorkerManager routing gaps for optional/unavailable segments and empty partitions

[xiangfu0]

Motivation

pinot-query-planner/src/main/java/org/apache/pinot/query/routing/WorkerManager.java still has TODOs for several unsupported routing cases in the multi-stage engine:

• optional segments
• unavailable segments
• partitions with no segments
• workers whose assigned partitions all have no segments

These are correctness and resiliency gaps in stage assignment, especially for hybrid or partially unavailable tables.

Scope

• Define expected routing behavior for each unsupported case.
• Add planner and runtime tests for replicated and partitioned leaf stages.
• Implement the missing routing and metadata handling, or fail with clearer behavior if a case must stay unsupported.

Notes

Observed on upstream/master on April 15, 2026.