Fix NPE because of thread of DriverScheduler was not closed immediately when stop DN

[Wei-hao-Li]

Cause:

1. DriverScheduler.stop() returns before driver threads are fully gone.
2. During that gap, MPP teardown can deregister FI memory accounting first.
3. A still-running driver cleanup path then calls MemoryPool.free(...) using the same IDs.
4. free() hits a removed map entry (null) and throws IllegalArgumentException wrapping NPE.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 40.12%. Comparing base (cee5fbb) to head (cbd90d7).
⚠️ Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
...ueryengine/execution/schedule/DriverScheduler.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #17616      +/-   ##
============================================
+ Coverage     40.09%   40.12%   +0.02%     
  Complexity     2554     2554              
============================================
  Files          5177     5177              
  Lines        348659   348758      +99     
  Branches      44591    44609      +18     
============================================
+ Hits         139794   139932     +138     
+ Misses       208865   208826      -39     

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