fix: validate EOF for chunked h1 responses

[mcollina]

Summary

• validate HTTP/1.1 response completeness with llhttp_finish() on EOF-like socket termination
• preserve the existing Content-Length mismatch error behavior
• add regression coverage for truncated chunked responses in Client and fetch()

Details

Undici previously treated EOF on non-keepalive responses as an implicit successful message completion. That is correct for EOF-delimited responses, but not for chunked responses, which must receive the terminating 0\r\n\r\n chunk.

This change routes the EOF/close/ECONNRESET path through llhttp_finish() so llhttp can reject incomplete chunked responses with an invalid EOF state instead of completing the response body successfully.

Testing

• node --test test/parser-issues.js test/content-length.js
• npx eslint lib/dispatcher/client-h1.js test/parser-issues.js

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.44068% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.27%. Comparing base (0b06a16) to head (ab6aae5).

Files with missing lines	Patch %	Lines
lib/dispatcher/client-h1.js	86.44%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5273      +/-   ##
==========================================
- Coverage   93.28%   93.27%   -0.01%     
==========================================
  Files         110      110              
  Lines       36384    36427      +43     
==========================================
+ Hits        33939    33978      +39     
- Misses       2445     2449       +4     

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[trivikr]

Needs rebase to include node 26 FFI build from https://www,github.com/nodejs/undici/pull/5275

[mcollina]

I think we can land without