maint: Refactor Channel Loader

[jjerphan]

Description

Mostly clarify the implementation and also add C++ tests.

Type of Change

• Bugfix
• Feature / enhancement
• CI / Documentation
• Maintenance

Checklist

• My code follows the general style and conventions of the codebase, ensuring consistency
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have run pre-commit run --all locally in the source folder and confirmed that there are no linter errors.
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes

[codecov]

Codecov Report

❌ Patch coverage is 32.35294% with 46 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.43%. Comparing base (7603af9) to head (75872c7).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
libmamba/src/api/channel_loader.cpp	32.35%	46 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4169      +/-   ##
==========================================
+ Coverage   52.15%   52.43%   +0.28%     
==========================================
  Files         239      239              
  Lines       28803    28803              
  Branches     3029     3026       -3     
==========================================
+ Hits        15021    15102      +81     
+ Misses      13779    13698      -81     
  Partials        3        3              

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[JohanMabille]

LGTM when my last comment is adressed.