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]

✅ Deploy Preview for endearing-brigadeiros-63f9d0 canceled.

Name	Link
🔨 Latest commit	4cc15cf
🔍 Latest deploy log	https://app.netlify.com/projects/endearing-brigadeiros-63f9d0/deploys/683162d386efca000862c49b

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.46%. Comparing base (fc6f166) to head (4cc15cf).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1025      +/-   ##
==========================================
+ Coverage   48.51%   53.46%   +4.94%     
==========================================
  Files          51       52       +1     
  Lines        2092     2108      +16     
  Branches      241      243       +2     
==========================================
+ Hits         1015     1127     +112     
+ Misses       1040      942      -98     
- Partials       37       39       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.