test: use Undici matching version bundled with Node.js

[paulrobertlloyd]

Tests mocked HTTP requests using a MockAgent from undici@8, but CI runs on Node.js 24, whose built-in fetch() is powered by Undici 7. Node.js doesn’t expose its bundled Undici, and dispatchers aren’t compatible across major versions (see nodejs/undici#5341), so tests were passing by luck rather than design.

Changes

• Adds @indiekit-test/undici helper, which installs undici@7 and undici@8 as aliases and re-exports MockAgent, setGlobalDispatcher etc. from whichever matches process.versions.undici
• Updates mock agent helpers and tests to import from this helper, and removes the direct undici dependency
• Adds ESLint rule preventing undici from being imported directly
• Adds smoke tests that check the major versions match, and that built-in fetch() intercepted by MockAgent keeps response headers and decodes compressed bodies (the symptoms of a mismatch)
• Runs tests on Node.js 24 and 26

Notes

When a future Node.js release bundles a new major version of Undici, add an alias to helpers/undici/package.json. Until then, the helper throws with a message explaining what to do.