#12876: major test workflow: enhance coverage infrastructure

<!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR adds a dedicated `Test Coverage` GitHub Actions workflow that runs Vitest coverage and uploads `coverage/lcov.info` to Codecov, expands Vitest’s coverage output to include HTML reports, and documents the test/coverage commands and directory structure in README + `docs/testing.md`. It also adds a handful of test-focused ignore patterns (coverage/nyc/vitest artifacts) and introduces convenience `pnpm` scripts for running specific Vitest configs (unit/extensions/gateway) and a coverage UI mode. These changes plug into the existing Node/Vitest test runner (`scripts/test-parallel.mjs`) and CI, but the new workflow currently uses different Node/pnpm versions than the rest of the repo CI, and relies on an `nyc` CLI that is not installed- in this repository. <h3>Confidence Score: 3/5</h3> - This PR is close, but the new coverage workflow will fail or be non-deterministic until its toolchain and summary generation are fixed. - Most changes are documentation and configuration, but `.github/workflows/test-coverage.yml` currently runs with Node 20 + pnpm 8 (mismatching repo requirements) and uses `npx nyc` without a pinned dependency, which will break or introduce flaky CI behavior. - .github/workflows/test-coverage.yml <!-- greptile_other_comments_section --> _{(5/5) You can turn off certain types of comments like style [here](https://app.greptile.com/review/github)!} <!-- /greptile_comment -->