docs: Clarify nil http.Client usage has no timeout

[merchantmoh-debug]

This PR adds a documentation comment to the NewClient function in github/github.go.

The comment warns users that passing a nil httpClient results in the use of a default http.Client which has no timeout. This can lead to resource exhaustion or "slowloris" style issues in production environments. It recommends providing a custom http.Client with an appropriate timeout.

This change aligns with security best practices (Sentinel) by making the default behavior's risks explicit without introducing breaking changes

[google-cla]

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[gmlewis]

Thank you, @merchantmoh-debug.
LGTM.
Merging.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.44%. Comparing base (4456d12) to head (7409d8e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3910   +/-   ##
=======================================
  Coverage   92.44%   92.44%           
=======================================
  Files         203      203           
  Lines       14927    14927           
=======================================
  Hits        13799    13799           
  Misses        926      926           
  Partials      202      202           

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[gmlewis]

Sorry, I spoke too soon. I cannot merge this until the CLA is signed.
If you prefer to not sign the CLA, I will close the PR. Please let me know, @merchantmoh-debug.