fix: Adapt Shards base URL to handle channel identifiers

[jjerphan]

Description

This resolves some warnings when conda-forge's sharded repodata is used.

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 70.00000% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.86%. Comparing base (6db7f56) to head (ae83f27).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
libmamba/src/core/shards.cpp	68.75%	10 Missing ⚠️
libmamba/src/core/channel_context.cpp	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4210      +/-   ##
==========================================
- Coverage   53.40%   52.86%   -0.55%     
==========================================
  Files         239      239              
  Lines       29301    29334      +33     
  Branches     3123     3127       +4     
==========================================
- Hits        15648    15506     -142     
- Misses      13650    13825     +175     
  Partials        3        3              

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