chore: add vitest to devDependencies with config

Adds vitest as test runner for the MCP repo. Config targets
__tests__/**/*.test.ts and excludes vendor/ and node_modules/.

vitest.config.ts

@@ -0,0 +1,9 @@
+import { defineConfig } from 'vitest/config'
+
+export default defineConfig({
+  test: {
+    environment: 'node',
+    include: ['__tests__/**/*.test.ts'],
+    exclude: ['vendor/**', 'node_modules/**'],
+  },
+})