Timeout exceptions is not set clearly in query response.

Currently we don't see timeout exception in pinot query exceptions.
Sometimes we just see pure empty response, and sometime we see the message below, which shows no clue of this is timeout and users need to see Pinot server log to identify the real issue is timeout.

This also prevents client side to retry on certain exceptions.

```
{"errorCode":200,"message":"QueryExecutionError:\njava.lang.RuntimeException: Caught exception while running CombinePlanNode.\n\tat org.apache.pinot.core.plan.CombinePlanNode.run(CombinePlanNode.java:148)\n\tat org.apache.pinot.core.plan.InstanceResponsePlanNode.run(InstanceResponsePlanNode.java:38)\n\tat org.apache.pinot.core.plan.GlobalPlanImplV0.execute(GlobalPlanImplV0.java:45)\n\tat org.apache.pinot.core.query.executor.ServerQueryExecutorV1Impl.processQuery(ServerQueryExecutorV1Impl.java:213)\n\tat org.apache.pinot.core.query.scheduler.QueryScheduler.processQueryAndSerialize(QueryScheduler.java:152)\n\tat org.apache.pinot.core.query.scheduler.QueryScheduler.lambda$createQueryFutureTask$0(QueryScheduler.java:136)\n\tat java.util.concurrent.FutureTask.run(FutureTask.java:266)\n\tat java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)\n\tat shaded.com.google.common.util.concurrent.TrustedListenableFutureTask$TrustedFutureInterruptibleTask.runInterruptibly(TrustedListenableFutureTask.java:111)\n\tat shaded.com.google.common.util.concurrent.InterruptibleTask.run(InterruptibleTask.java:58)\n\tat shaded.com.google.common.util.concurrent.TrustedListenableFutureTask.run(TrustedListenableFutureTask.java:75)\n\tat java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)\n\tat java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)\n\tat java.lang.Thread.run(Thread.java:748)"}
```

Start by tracing timeout handling from QueryScheduler.processQuery through ServerQueryExecutorV1Impl.processQuery and the response serialization path. Compare the empty response and QueryExecutionError examples, then run the relevant query execution tests to verify that timeout failures are identifiable in the client response and suitable for retry decisions.

Written by the indexing model from the issue text.