Update vitest to v4.0.10 and fix test mocks for vitest 4.x compatibility.

- Updated vitest from ^3.2.4 to ^4.0.10 to match @vitest/coverage-v8 peer dependency requirement.
- Fixed McpdClient mock to use proper constructor function instead of arrow function, as required by vitest 4.x stricter mocking behavior.

Resolves dependency conflict where @vitest/coverage-v8@4.0.10 requires vitest@4.0.10 as peer dependency.

Author: peteski22