fix(retry): reject invalid resumed responses

[mcollina]

Summary

• validate resumed responses before generic status-code retry handling
• reject non-206 continuations after response bytes have already been forwarded
• cover both throwOnError modes and verify that continuation bytes are not appended downstream

Rationale

Once a response has been partially consumed, a retry can only continue it with a valid 206 Partial Content response. Previously, responses with a status code of 300 or greater were handled before the continuation checks, allowing an invalid continuation body to be forwarded to the existing downstream stream.

Tests

• npm run lint
• node --test test/retry-handler.js test/retry-handler2.js test/retry-agent.js test/retry-handler-controller-proxy.js test/client-retry-resume-backpressure.js test/http2-goaway-retry-body.js (56 passed)
• node --test test/issue-3934.js test/issue-5087.js test/issue-5137.js test/issue-3356.js test/issue-4691.js (8 passed)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.47%. Comparing base (5033aa1) to head (a3618df).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5724   +/-   ##
=======================================
  Coverage   93.47%   93.47%           
=======================================
  Files         110      110           
  Lines       38906    38908    +2     
=======================================
+ Hits        36366    36368    +2     
  Misses       2540     2540           

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