fix(test): make sure dl_url is a string

[Guts]

Example of failed job: https://github.com/qgis-deployment/qgis-deployment-toolbelt-cli/actions/runs/12087850915/job/33710161685

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-advanced-security]

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

@@           Coverage Diff           @@
##             main     #594   +/-   ##
=======================================
  Coverage   70.35%   70.35%           
=======================================
  Files          47       47           
  Lines        3188     3188           
  Branches      558      558           
=======================================
  Hits         2243     2243           
  Misses        747      747           
  Partials      198      198           

Flag	Coverage Δ
unittests	69.79% <ø> (ø)

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[jmkerloch]

LGTM !