Change: Replace Jest with Vitest and improve test infrastructure

• Replace Jest with Vitest for better ESM compatibility and Vite plugin testing
• Fix flaky tests by implementing proper port collision handling
• Each test now manages its own server instance instead of relying on a shared afterEach cleanup hook, eliminating race conditions and flaky test behavior
• Enable test parallelization
• Reduce CI noise and improve output clarity
• Optimize build process by directly calling tsc instead of spawning new pnpm instances
• Use much simpler eslint config

This is all in preparation of the Vite 6 update, which was impossible to debug before.