feat: implement api server, refactor benchmark, fix tests

- Installed express, cors, helmet, dotenv.
- Modified `src/index.js` to run the Express API server when the `--server` flag is present, and execute the benchmark logic otherwise.
- Maintained legacy `main()` benchmark logic.
- Fixed the `t.end is not a function` error in `tests/test.js` by removing unsupported `t.end()` calls in the native test runner.
- Added API endpoints `/health` and `/v1/chat/completions` (with mocked output).
- Created `tests/api.test.js` to run supertest queries against the new server structure.
- Updated `package.json` with accurate dependencies, scripts (`npm start`, `npm dev`), and tests.

Co-authored-by: shenald-dev <245350826+shenald-dev@users.noreply.github.com>

Author: google-labs-jules[bot]

.gitignore

@@ -0,0 +1 @@
+node_modules/