feat: Add unit tests for plugins

[cmendible]

Description

This pull request adds comprehensive unit tests for the internal plugins system, YAML plugin loading and validation, the carbon emissions scanner, and the region availability scanner. These tests improve coverage for plugin registration, plugin discovery, YAML validation, emissions report formatting, and region SKU logic, ensuring correctness and reliability of core plugin and scanner features.

Issue reference

We strive to have all PR being opened based on an issue, where the problem or feature have been discussed prior to implementation.

Please reference the issue this PR will close: #909

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Unit tests passing

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.44444% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 30.62%. Comparing base (2436646) to head (ef1945d).

Files with missing lines	Patch %	Lines
internal/scanners/plugins/carbon/emissions.go	92.85%	2 Missing ⚠️
internal/scanners/plugins/sqlesu/sqlesu.go	97.61%	1 Missing ⚠️
internal/scanners/plugins/zone/mapping.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #910      +/-   ##
==========================================
+ Coverage   26.11%   30.62%   +4.51%     
==========================================
  Files          96       96              
  Lines        7429     7395      -34     
==========================================
+ Hits         1940     2265     +325     
+ Misses       5374     5017     -357     
+ Partials      115      113       -2     

Flag	Coverage Δ
unittests	30.62% <94.44%> (+4.51%)	⬆️

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