docs: add security warning for strictContentLength option

[mcollina]

Summary

• Added security warning to strictContentLength documentation in Client.md and H2CClient.md
• Documents risks of disabling this option: HTTP Request Smuggling attacks, cache poisoning, credential hijacking, and bypassing security controls
• Recommends only disabling in controlled environments with trusted request sources

Test plan

• Verify documentation renders correctly in markdown

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.87%. Comparing base (d560767) to head (1fce116).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4726   +/-   ##
=======================================
  Coverage   92.86%   92.87%           
=======================================
  Files         109      109           
  Lines       33809    33818    +9     
=======================================
+ Hits        31398    31407    +9     
  Misses       2411     2411           

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[Uzlopak]

LGTM