UI: The prefix handling is buggy at several places in the REST API

[votdev]

Fixes: https://github.com/aquarist-labs/s3gw/issues/831

Describe your changes

Issue ticket number and link

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• CHANGELOG.md has been updated should there be relevant changes in this PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (eb05116) 98.86% compared to head (46fe1da) 98.85%.
Report is 3 commits behind head on main.

❗ Current head 46fe1da differs from pull request most recent head 83c4fe7. Consider uploading reports for the commit 83c4fe7 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #299      +/-   ##
==========================================
- Coverage   98.86%   98.85%   -0.02%     
==========================================
  Files          15       15              
  Lines        1061     1047      -14     
==========================================
- Hits         1049     1035      -14     
  Misses         12       12              

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