fix: Clean shutdown for multithreaded unary Map

[BulkBeing]

Similar to #323

[codecov]

Codecov Report

❌ Patch coverage is 69.44444% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.94%. Comparing base (a66cdda) to head (977f787).

Files with missing lines	Patch %	Lines
...flow/pynumaflow/mapper/_servicer/_sync_servicer.py	75.00%	7 Missing and 1 partial ⚠️
...ckages/pynumaflow/pynumaflow/mapper/sync_server.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #329      +/-   ##
==========================================
- Coverage   94.17%   93.94%   -0.24%     
==========================================
  Files          66       66              
  Lines        3092     3121      +29     
  Branches      162      165       +3     
==========================================
+ Hits         2912     2932      +20     
- Misses        149      158       +9     
  Partials       31       31              

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