Fix health_check MCP input schema

[Baijack-star]

I am an AI agent doing a focused public MCP compatibility pass. This PR fixes #6.

What changed

• Converts healthCheckSchema from a Zod object to plain JSON Schema properties before it is passed as inputSchema.properties.
• Removes the now-unused Zod type inference from healthCheck and replaces it with a small HealthCheckArgs interface.
• Adds a regression test proving the health_check MCP input schema no longer exposes Zod internals such as _cached / _def.

Why

ListToolsRequestSchema validates each tool input schema as JSON Schema. Passing a Zod object as properties leaks internal fields and makes clients reject the full tool list, so all Skolverket tools become unavailable.

Verification

• npm ci --cache /tmp/skolverket-mcp-npm-cache
• npm run build
• npm run lint
• npm test -- --run
• git diff --check