fix(deduplicate): make terminal callbacks exception-safe

[mcollina]

Make deduplication terminal callbacks exception-safe. Remove completed requests from the pending map before notifying downstream handlers, forward completion errors to the corresponding handler, and ensure one throwing callback does not prevent other callers from completing.

Preserve retry eligibility when a waiting handler rejects completion, and handle deferred completion after a paused caller resumes. Add regression coverage for callback failures, reentrant dispatch, and paused callers.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.09091% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.57%. Comparing base (bfea020) to head (ad30f0a).

Files with missing lines	Patch %	Lines
lib/handler/deduplication-handler.js	89.09%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5838      +/-   ##
==========================================
+ Coverage   93.53%   93.57%   +0.04%     
==========================================
  Files         110      110              
  Lines       39773    39790      +17     
==========================================
+ Hits        37200    37234      +34     
+ Misses       2573     2556      -17     

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