fix(h2): validate response content-length

[mcollina]

Validate response Content-Length against received DATA bytes in the HTTP/2 client. Truncated and oversized bodies now fail with the existing response length mismatch error while HEAD and 304 semantics are preserved. Includes fetch regression coverage for mismatches and session reuse.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.32258% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.50%. Comparing base (fb3d786) to head (a5190f7).

Files with missing lines	Patch %	Lines
lib/dispatcher/client-h2.js	90.32%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5816      +/-   ##
==========================================
- Coverage   93.50%   93.50%   -0.01%     
==========================================
  Files         110      110              
  Lines       39557    39585      +28     
==========================================
+ Hits        36987    37013      +26     
- Misses       2570     2572       +2     

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