fix(devtool): handle trailing slash in URL and improve error message

[mafredri]

A trailing slash in the URL caused double-slash paths like "//json/version",
which failed with the misleading error "could not resolve IP for 127.0.0.1".

Now we strip trailing slashes and show the actual endpoints tried on failure.

Fixes #152

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 39.7%. Comparing base (3b1f600) to head (688b926).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #155     +/-   ##
=======================================
+ Coverage   39.5%   39.7%   +0.2%     
=======================================
  Files         25      25             
  Lines       2322    2325      +3     
=======================================
+ Hits         918     924      +6     
+ Misses      1363    1362      -1     
+ Partials      41      39      -2     

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