fix(scheduler): Provide blocked future when scheduler is blocked on placement (#26494)

## Description

Provide blocked future when scheduler is blocked on placement

## Motivation and Context
<!---Why is this change required? What problem does it solve?-->
<!---If it fixes an open issue, please link to the issue here.-->

To avoid unnecessary scheduling delays

When scheduler is blocked on placement but the placement future is not
provided the query scheduler ends up waiting for 1s unnecessarily.

## Impact

Improved scheduling latency for bucketed execution and single node
execution modes for queries with more than a single TableScan

## Test Plan

Assertion + CI

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in particular [code
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- [x] PR description addresses the issue accurately and concisely. If
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## Release Notes
Please follow [release notes
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and fill in the release notes below.

```
== NO RELEASE NOTE ==
```

Author: arhimondr

presto-main-base/src/main/java/com/facebook/presto/execution/scheduler/SourcePartitionedScheduler.java

@@ -258,6 +258,7 @@ else if (scheduleGroup.pendingSplits.isEmpty()) {
             Multimap<InternalNode, Split> splitAssignment = ImmutableMultimap.of();
             if (!scheduleGroup.pendingSplits.isEmpty()) {
                 if (!scheduleGroup.placementFuture.isDone()) {
+                    overallBlockedFutures.add(scheduleGroup.placementFuture);
                     anyBlockedOnPlacements = true;
                     continue;
                 }
@@ -375,6 +376,8 @@ else if (scheduleGroup.pendingSplits.isEmpty()) {
             blockedReason = anyBlockedOnPlacements ? SPLIT_QUEUES_FULL : NO_ACTIVE_DRIVER_GROUP;
         }
 
+        verify(!overallBlockedFutures.isEmpty() || blockedReason == NO_ACTIVE_DRIVER_GROUP, "overallBlockedFutures is expected to be not empty when blocked on placement or splits");
+
         overallBlockedFutures.add(whenFinishedOrNewLifespanAdded);
         return ScheduleResult.blocked(
                 false,