Migrate sequentialthinking and filesystem servers from Jest to Vitest

- Replace Jest dependencies with Vitest and @vitest/coverage-v8
- Update test scripts to use 'vitest run --coverage'
- Create vitest.config.ts for both servers with node environment and coverage settings
- Update all test files:
  - Change imports from '@jest/globals' to 'vitest'
  - Replace jest.mock() with vi.mock()
  - Replace jest.fn() with vi.fn()
  - Update mock clearing/restoring to use vi methods
- Remove jest.config.cjs files
- All 151 tests passing (24 in sequentialthinking, 127 in filesystem)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

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