test: cover tool metadata diffs

[koriyoshi2041]

Summary

• Add a focused regression test for compareProbeResults()
• Verify tool description changes are reported under the tools endpoint diff
• Verify nested input schema description changes are also reported under the tools endpoint diff

Why

Tool descriptions and schema descriptions are part of an MCP server's public interface. This test makes that behavior explicit so future diff changes do not accidentally stop reporting metadata drift.

Validation

• npm test -- --runInBand src/__tests__/diff.test.ts
• npm run typecheck
• npm run lint
• npm run format:check
• npm run check