maint: Adapt Shards ownership model

[jjerphan]

Description

Follow-up of #4162 (comment).

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 66.00000% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.27%. Comparing base (3c948a4) to head (27d4c5a).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
libmamba/src/core/shard_traversal.cpp	76.74%	10 Missing ⚠️
libmamba/src/api/channel_loader.cpp	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4177      +/-   ##
==========================================
- Coverage   52.76%   52.27%   -0.50%     
==========================================
  Files         239      239              
  Lines       29065    29071       +6     
  Branches     3075     3076       +1     
==========================================
- Hits        15337    15197     -140     
- Misses      13725    13871     +146     
  Partials        3        3              

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[JohanMabille]

The mamba-content-trust and windows tests failure are unrelated to this PR.