Surface HTTP/2 response resets after buffered data

[quinnj]

Summary

• preserve buffered HTTP/2 response DATA when a stream reset arrives
• surface the terminal stream reset after the buffered bytes are drained
• keep a response that already completed with END_STREAM when a later reset races in
• clean up the body callback and stream registration without poisoning the shared HTTP/2 connection

Root cause

body_read! treated stream_done as clean EOF without checking the terminal stream error recorded by the read loop. A response without Content-Length could therefore return a truncated prefix as a successful complete body after RST_STREAM.

The reset handler also needed to distinguish normal response completion from error-driven stream_done. A late reset must not discard a response that already completed normally.

Validation

• julia --startup-file=no --project=. test/http2_client_tests.jl
  • partial-reset matrix: 27/27
  • completed-response late-reset case: 7/7
  • completed multiplexed response survives a sibling connection failure: 7/7
  • terminal GOAWAY error details: 9/9
• julia --startup-file=no --project=. test/http_client_transport_tests.jl — 106/106
• julia --startup-file=no --project=. test/http_retry_tests.jl — 93/93
• raw HTTP/2 peer probes cover partial DATA with and without Content-Length, complete responses followed by NO_ERROR or CANCEL, and a completed stream sharing a failed connection
• independent Claude Desktop review with Fable 5 and Extra effort: CLEAN; restored-branch mutation fails only the new multiplexed regression

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.38%. Comparing base (90c2359) to head (2c824e0).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1343      +/-   ##
==========================================
+ Coverage   88.36%   88.38%   +0.01%     
==========================================
  Files          30       30              
  Lines       11915    11923       +8     
==========================================
+ Hits        10529    10538       +9     
+ Misses       1386     1385       -1     

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