Add configuration validation

[jraddaoui]

• Verify that required fields have values
• Verify that MaxConcurrentSessions > 0
• Add comments describing config fields
• Add tests for invalid configuration

[codecov]

Codecov Report

Attention: Patch coverage is 96.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 27.59%. Comparing base (e966ea2) to head (0225937).
Report is 1 commits behind head on main.

Files	Patch %	Lines
internal/config/config.go	96.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #10      +/-   ##
==========================================
+ Coverage   24.67%   27.59%   +2.92%     
==========================================
  Files          16       16              
  Lines         608      627      +19     
==========================================
+ Hits          150      173      +23     
+ Misses        438      436       -2     
+ Partials       20       18       -2     

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