test: limit cache-tests workers on Windows

[mcollina]

Summary

• run cache-test environment workers serially on Windows to avoid intermittent worker termination
• keep existing parallelism on other platforms
• add CACHE_TEST_WORKER_CONCURRENCY for validation/debugging

Fixes: #5199

Testing

• npm run lint -- test/cache-interceptor/cache-tests.mjs
• npm run test:cache-tests
• CACHE_TEST_WORKER_CONCURRENCY=1 npm run test:cache-tests

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.23%. Comparing base (bd98954) to head (5087839).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5309   +/-   ##
=======================================
  Coverage   93.23%   93.23%           
=======================================
  Files         110      110           
  Lines       36575    36575           
=======================================
+ Hits        34101    34102    +1     
+ Misses       2474     2473    -1     

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