Add tests for Everything Server

[olaservo]

Is your feature request related to a problem? Please describe.

Edit, see: https://github.com/modelcontextprotocol/servers/issues/3080 since I think this will be a prerequisite to making testing possible.

Everything Server is arguably the most important reference server here but it still needs test coverage. I've been saving it for last since it might need refactoring and others are actively making changes to it.

Describe the solution you'd like

Vitest coverage

Describe alternatives you've considered

Relying on manual testing

Additional context