test: improve proxy route test coverage

[jescalada]

This PR improves test coverage for /src/proxy/routes. Overall project coverage should increase by ~5%.

Originally, I wanted to improve coverage for /plugin.ts, however I've been having problems with the tests not executing due to an ESM interop problem when using the plugin-loader library.

I added a skip on all the problematic tests in plugin.test.js, and kept the ones that do work.

I tried a few workarounds (such as executing with regular mocha + tsx loader, running with vitest) without any luck. The only working solution I found so far is to upgrade to Node 22.

[netlify]

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Name	Link
🔨 Latest commit	c6ed94c
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.62%. Comparing base (96c4754) to head (c6ed94c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1025      +/-   ##
==========================================
+ Coverage   73.22%   75.62%   +2.39%     
==========================================
  Files          55       55              
  Lines        2241     2244       +3     
  Branches      249      251       +2     
==========================================
+ Hits         1641     1697      +56     
+ Misses        570      517      -53     
  Partials       30       30              

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[JamieSlome]

LGTM! 🍰