fix(retry): settle responses after connection close

[mcollina]

Ensure retry responses that finish while their controller is paused are retained until the retry policy completes. This lets retries advance after a peer closes the connection and preserves the final response body when retries are exhausted.

Adds concurrent coverage for connection-closing retry responses.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.66667% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.46%. Comparing base (fb3d786) to head (18ae0da).

Files with missing lines	Patch %	Lines
lib/handler/retry-handler.js	66.66%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5815      +/-   ##
==========================================
- Coverage   93.50%   93.46%   -0.04%     
==========================================
  Files         110      110              
  Lines       39557    39599      +42     
==========================================
+ Hits        36987    37013      +26     
- Misses       2570     2586      +16     

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