[AQE-PR-4] Late-bound materialized routing - #72

Summary

Bind completed partition handles to consumers immediately before serialization and dispatch.

producer handles
     |
validate P x M coverage
     |
partition p -> consumer worker p
     |
serialize + dispatch
     |
read assigned local/remote files

Worker counts stay fixed. Runtime partition coalescing is a follow-up, not part of this PR.

Stack

Builds on AQE-PR-3: #71
Roadmap: apache#19485

Validation

Local routing/SerDe tests and real-RPC GROUP BY queries (normal and empty output) passed; style/license checks passed.

Reproduce full-query real-RPC tests

From this PR's checkout, with JDK 25+. The fixture starts its own servers; no external cluster is needed. Two query servers execute real GROUP BY operators, gRPC dispatch and materialized file reads; parallelism stays fixed:

./mvnw -B -ntp -pl pinot-query-runtime -am \
  '-Dtest=QueryRunnerTest#testMaterializedStagedQuery' \
  -Dsurefire.failIfNoSpecifiedTests=false \
  -Dmaven.compiler.showDeprecation=false test

Local report: 2 cases passed:

• Nonempty GROUP BY ... ORDER BY: schema and rows match the nonmaterialized baseline; late-bound input handles are present.
• The same query filtered to empty output: the empty result/schema still matches baseline through the complete staged path.

These integrate materialized exchange (PR2) + staged dispatch (PR3) + late binding (PR4). They are also rerun as regression cases in PR5, not counted as new PR5 features.

Test source · Results by phase · Adaptive extension / AQE-PR-5.