fix: use canonical names inside TUF fetcher

[kipz]

Previous PR (#142) was only a partial fix because targets URL parsing was still broken.

This PR moves parsing logic into the Fetcher constructor so that we are always using canonical name matching regardless of how TUF OCI Fetch is parameterized.

Also add some more tests

[codecov]

Codecov Report

Attention: Patch coverage is 94.73684% with 1 line in your changes missing coverage. Please review.

Project coverage is 66.89%. Comparing base (bada1df) to head (3b7536c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/tuf/tuf.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #144      +/-   ##
==========================================
+ Coverage   66.73%   66.89%   +0.15%     
==========================================
  Files          40       40              
  Lines        2324     2332       +8     
==========================================
+ Hits         1551     1560       +9     
+ Misses        505      504       -1     
  Partials      268      268              

Flag	Coverage Δ
unittests	66.89% <94.73%> (+0.15%)	⬆️

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[jonnystoten]

LGTM!