docs(indexer): document test runner conventions

[alan-provable]

Summary

• document the S03 indexer test runner conventions and current pure-logic test inventory
• run SubQuery codegen before the package test build so clean checkouts generate ignored src/types
• declare tslib, required by the TypeScript importHelpers setting during SubQuery build

Verification

• npx bun run --cwd apps/s03-indexer test
• npx prettier --check apps/s03-indexer/package.json apps/s03-indexer/README.md

Closes #257

[vercel]

@alan-provable is attempting to deploy a commit to the Ijai's projects Team on Vercel.

A member of the Team first needs to authorize it.

[alan-provable]

Quick check note: the only failing status I can see is Vercel asking a SO4 team member to authorize the preview deployment for this fork. The local validation in the PR body passed on my side (npx bun run --cwd apps/s03-indexer test and the focused Prettier check). Happy to re-run anything maintainers want after the preview authorization gate is cleared.