fix: Properly join URI paths in PlPath

[coastalwhite]

This also adds some tests to make sure that the behavior is somewhat consistent now.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.77%. Comparing base (54d39f0) to head (5089ed5).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23350      +/-   ##
==========================================
+ Coverage   80.75%   80.77%   +0.01%     
==========================================
  Files        1644     1644              
  Lines      221756   221788      +32     
  Branches     2786     2786              
==========================================
+ Hits       179088   179145      +57     
+ Misses      42006    41981      -25     
  Partials      662      662              

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