fix: remove System.exit from JVM shutdown hooks

[KeranYang]

to prevent shutdown deadlock, for both source transformer and mapper. Also added a unit test to reproduce the issue and verify the fix.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 26 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@be343da). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
.../main/java/io/numaproj/numaflow/mapper/Server.java	0.00%	13 Missing ⚠️
...io/numaproj/numaflow/sourcetransformer/Server.java	0.00%	13 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #225   +/-   ##
=======================================
  Coverage        ?   61.17%           
  Complexity      ?      598           
=======================================
  Files           ?      157           
  Lines           ?     3668           
  Branches        ?      271           
=======================================
  Hits            ?     2244           
  Misses          ?     1247           
  Partials        ?      177           

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[vaibhavtiwari33]

Although async, should we add the fix here as well?

[KeranYang]

Good catch Vaibhav, thanks! Since we fixed the shutdown hook to stop the AKKA system, this line is actually fixed because System.exit(0); triggers the shutdown hook. I updated the comments.