Upgrade to TypeScript 5.9, ESLint 9 (flat config), Prettier 3, and migrate Jest to Vitest

[Copilot]

Modernizes tooling to latest stable versions: TypeScript 4.5→5.9, ESLint 8→9, typescript-eslint 5→8, Prettier 2→3, and replaces Jest with Vitest.

Package Updates

• TypeScript 4.5.5 → 5.9.3
• ESLint 8.9.0 → 9.21.0
• @typescript-eslint/{parser,eslint-plugin} 5.12.0 → 8.54.0
• Prettier 2.5.1 → 3.8.1
• Vitest 4.0.18 (new, replaces Jest)
• Other: esbuild, husky, lint-staged, @types/node

ESLint 9 Flat Config Migration

Migrated from legacy .eslintrc.js to ESLint 9's flat config format:

// eslint.config.js
const tseslint = require('@typescript-eslint/eslint-plugin');
const tsparser = require('@typescript-eslint/parser');

module.exports = [
    { ignores: ['dist/**', 'node_modules/**', 'coverage/**'] },
    js.configs.recommended,
    {
        files: ['**/*.{js,ts}'],
        languageOptions: { parser: tsparser, /* ... */ },
        plugins: { '@typescript-eslint': tseslint },
        rules: { /* preserved existing rules */ }
    }
];

Removed deprecated .eslintrc.js and .eslintignore.

Jest → Vitest Migration

// vitest.config.ts
export default defineConfig({
    test: {
        globals: true,
        environment: 'node',
        include: ['tests/**/*.{test,spec}.{ts,js}']
    },
    resolve: {
        alias: { '@': './src', '@tests': './tests' }
    }
});

• Removed jest.config.js, jest, ts-jest, @types/jest
• Updated tsconfig.json types: jest → vitest/globals
• Updated test snapshots to Vitest format (cosmetic: Array [ → [, Object { → {)
• Changed npm scripts: jest tests → vitest run

Minor Changes

• Applied Prettier 3 formatting (trailing commas, minor whitespace)
• Removed unused ESLint disable directives flagged by new parser

Original prompt

refactor to use the latest package versions for eslint and typescript. make sure to update the eslint and prettier configs to use the latest. also install vitest package as a dev dep and convert the jest config to vitest config. take as long as you need.

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