Fix Base64Url encoding and harden authentication input validation

[kailash-b]

Changes

This pull request includes three distinct improvements to the Auth0.NET SDK:

1. Improved Validation for ForwardedForIp

• Added validation for the ForwardedForIp header in authentication API token requests
• Throws ArgumentException with descriptive message if an invalid IPv4 or IPv6 address is provided

2. Refactored ParseQuotaLimit to Handle Edge Cases

• Improved the ParseQuotaLimit() method in Extensions.cs to gracefully handle malformed headers instead of throwing exceptions

3. Fixed Base64 URL Encoding

• Corrected the Base64UrlEncode() method in Utils.cs to properly implement RFC 4648 Base64 URL encoding.

References

Testing

• This change adds unit test coverage

Checklist

• I have read the Auth0 general contribution guidelines

• I have read the Auth0 Code of Conduct

• All existing and new tests complete without errors

[codecov]

Codecov Report

❌ Patch coverage is 97.61905% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.35%. Comparing base (e024a15) to head (611915d).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Auth0.Core/Extensions.cs	96.29%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #919      +/-   ##
==========================================
+ Coverage   79.18%   79.35%   +0.17%     
==========================================
  Files         458      458              
  Lines        5932     5957      +25     
  Branches      278      282       +4     
==========================================
+ Hits         4697     4727      +30     
+ Misses       1140     1137       -3     
+ Partials       95       93       -2     

Flag	Coverage Δ
authIntTests	30.68% <76.19%> (+0.17%)	⬆️
mgmtIntTests	60.61% <7.14%> (-0.21%)	⬇️
unittests	9.29% <96.66%> (+0.36%)	⬆️

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