Migrate from Mocha to Vitest

[2nofa11]

Summary

Migrates the test framework from Mocha to Vitest to significantly improve test execution performance.

Closes #2787

Changes Made

060f779 - Initial Migration (Automated)

• Applied Codemod.com Mocha/Vitest recipe

3b667b0 - Manual Configuration & Integration Fixes

• Added vitest.config.ts with comprehensive test configuration:
  • Coverage provider set to v8
  • Configured include/exclude patterns
• Fixed integration tests to address Vitest worker limitations (vitest#1436):
  • Replaced process.chdir() usage with cwd option in cp.execSync() calls
  • Updated tests/integrations/eslint-plugin-import.js
  • Updated tests/integrations/flat-config.js
  • Replaced before/after hooks with beforeAll

ec06a3e - Global API Configuration

• Updated eslint.config.mjs to use vitest globals instead of mocha

Migration Details

• Test Coverage: All 11,059 tests pass without modification
• Compatibility: Maintains ESLint RuleTester compatibility
• Performance: 67% faster execution (74s → 24s)
• Configuration: Comprehensive vitest.config.ts with:
  • V8 coverage provider
  • HTML and LCOV reporting
  • Proper include/exclude patterns
  • Global API configuration

Test Plan

• All existing tests pass (11,059 tests)
• Integration tests work correctly
• Coverage reporting functional
• Performance improvement verified
• CI/CD pipeline validation needed

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Latest commit: ec06a3e

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[2nofa11]

Migrating to Vitest resulted in a Flaky test, because process.chdir() is not supported by Vitest (vitest-dev/vitest#1436).
So I substituted execSync({ cwd }).