Fix GASNetEx Injector Quiescence Race

[lightsighter]

Fix a GASNet-EX shutdown race caused by tracking concurrent injector workers with a boolean. When one worker finished before another, it could clear work_active prematurely, allowing network quiescence to be reported while an xpair was still in use.

Replace the boolean with an active-worker counter so every concurrent push_packets() invocation remains visible to quiescence detection.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 30.41%. Comparing base (3c47fec) to head (dc59db4).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #470    +/-   ##
========================================
  Coverage   30.41%   30.41%            
========================================
  Files         199      199            
  Lines       41282    41282            
  Branches    14810    14783    -27     
========================================
  Hits        12554    12554            
+ Misses      27759    27398   -361     
- Partials      969     1330   +361     

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