fix(runner): properly encode Uint8Array body in annotations

[Livan-pro]

Description

This PR fixes an issue with the incorrect encoding of the Uint8Array body in test annotations. The root cause is that the table for base64 encoding is missing characters at indices 62 and 63. This bug causes "undefined" to appear in the base64-encoded body. I've figured this out while trying to view image annotations in an HTML report. They were not loading.
Reproducible in Vitest v4.0.7 (and I believe since the introduction of annotations in v3.2.0).

I didn't find a reported issue on GitHub.

BTW, I have a small question. Is there a reason to use for loops to populate the table instead of a simple const table = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'? 🤔

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. If the feature is substantial or introduces breaking changes without a discussion, PR might be closed.
• Ideally, include a test that fails without this PR but passes with it.
• Please, don't make changes to pnpm-lock.yaml unless you introduce a new test example.
• Please check Allow edits by maintainers to make review process faster. Note that this option is not available for repositories that are owned by Github organizations.

Tests

• Run the tests with pnpm test:ci.

Documentation

• If you introduce new functionality, document it. You can run documentation with pnpm run docs command.

Changesets

• Changes in changelog are generated from PR name. Please, make sure that it explains your changes in an understandable manner. Please, prefix changeset messages with feat:, fix:, perf:, docs:, or chore:.

[netlify]

✅ Deploy Preview for vitest-dev ready!

Built without sensitive environment variables

Name	Link
🔨 Latest commit	aa2d45b
🔍 Latest deploy log	https://app.netlify.com/projects/vitest-dev/deploys/690c636e0644b50008db6a6c
😎 Deploy Preview	https://deploy-preview-8951--vitest-dev.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[sheremet-va]

Thank you!