chore(ci): Upload test results to action artifacts

[niclimcy]

Summary

Reduce push event noise on Discord

Checklist

Making changes

• Follows Inkdex's Contributing Guidelines.

For updates to existing extensions

• Bumped the version value in pbconfig.ts for each modified extension.

For new extensions

• Generated tests with npx paperback-cli test --generate EXTENSION_NAME.

Testing changes

• npm run conformance passes.
• npm test -- EXTENSION_NAME passes.
• Bundled the extension and verified it works in the Paperback app.

Committing changes

• Commit messages follow the existing convention (type(Scope): summary, e.g. fix(EXTENSION_NAME): ...).

AI assistance

Pick one:

• This PR contains no AI-assisted changes.
• This PR is AI-assisted. I manually reviewed every change and added an Assisted-by: AGENT_NAME:MODEL_VERSION trailer to each AI-assisted commit.