test: migrate from AVA to Vitest and remove test macros

[sudo-suhas]

Summary

This PR migrates the test suite from AVA to Vitest and eliminates all test macros by expanding them into explicit, self-documenting test cases. This improves developer experience with watch mode, built-in coverage reporting, and makes each test independently readable.

Key Changes

Dependencies and Configuration

• ✅ Replace AVA with Vitest for test execution
• ✅ Remove NYC in favor of Vitest's built-in coverage (@vitest/coverage-v8)
• ✅ Remove eslint-plugin-ava and add eslint-plugin-vitest
• ✅ Create vitest.config.js with ES Modules support, parallel execution, and coverage configuration
• ✅ Configure coverage to generate html, lcov, and text reports in a single test run

Test Migration (153 test files transformed)

• ✅ Convert all test files from AVA syntax to Vitest syntax
• ✅ Replace AVA's t.deepEqual() with expect().toEqual()
• ✅ Replace AVA's t.truthy() with expect().toBeTruthy()
• ✅ Replace AVA's t.throws() with expect().toThrow()
• ✅ Use explicit imports: import { test, expect, describe } from 'vitest'

Test Macro Elimination

• ✅ Remove test/_macros.js entirely (contained 6 test macro patterns)
• ✅ Expand setsAggType macro into explicit aggregation type tests
• ✅ Expand validatedCorrectly macro into table-driven validation tests
• ✅ Expand makeSetsOptionMacro generated tests into explicit option tests
• ✅ Expand illegalCall macro into explicit error tests with toThrow()
• ✅ Expand illegalParamType macro into explicit TypeError tests
• ✅ All expanded tests follow table-driven patterns for maintainability

Test Utilities

• ✅ Create test/testutil/ package for shared test helper functions
• ✅ Add type checking utilities for parameter validation
• ✅ Export all utilities from test/testutil/index.js
• ✅ Reuse existing recursiveToJSON from src/core/util.js

Package Scripts

• ✅ Update test:src: vitest run --coverage (single-step execution)
• ✅ Remove report script (now redundant with Vitest)
• ✅ Remove coverage script (now redundant with Vitest)
• ✅ Add test:watch: vitest for local development with watch mode
• ✅ Keep test:typedef unchanged for TypeScript type checking

ESLint Configuration

• ✅ Remove plugin:ava/recommended from test/.eslintrc.yml
• ✅ Add plugin:vitest/recommended to test/.eslintrc.yml
• ✅ Update plugins array to use vitest instead of ava

CI/CD

• ✅ Update .github/workflows/build.yml to use npm test instead of separate npm run coverage
• ✅ Maintain testing on Node.js 20.x, 22.x, 24.x
• ✅ Preserve Coveralls integration with lcov format

Bug Fixes

• ✅ Remove src/suggesters/phase-suggester.js (incorrectly named file, should be phrase-suggester.js which already exists)

Documentation

• ✅ Update README.md with new test commands using Vitest
• ✅ Update CONTRIBUTING.md with Vitest test instructions
• ✅ Add test/testing-guidelines.mdc for test writing guidelines

Testing

All 153 test files have been transformed and are passing. Coverage levels are maintained at the same levels as before the migration.

To run tests locally:

npm test              # Run all tests
npm run test:watch    # Watch mode for development
npm run test:src      # Run tests with coverage

Benefits

1. Better Developer Experience: Watch mode with instant feedback
2. Built-in Coverage: No need for separate NYC configuration
3. Self-Documenting Tests: All macro abstractions replaced with explicit test cases
4. Modern Tooling: Vitest is actively maintained and faster than AVA
5. Cleaner Test Code: Table-driven patterns instead of opaque macros

Breaking Changes

None for library users. This only affects the development/testing environment.