Added healthcheck endpoint

[JoshBragg-Cohere]

When using NorthMCPServer directly a healthcheck endpoint at /health will be enabled by default. This is primarily targeted to be used by k8s to ensure MCP is healthy.

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Note

Medium Risk
Adds a new unauthenticated /health route enabled by default, which slightly increases public surface area and could conflict with user-defined routes if they already use /health. The implementation is small but changes default server behavior.

Overview
NorthMCPServer now registers a built-in GET /health endpoint returning plain-text OK, enabled by default and bypassing auth; a new health_check=False constructor flag disables it.

Examples and docs are updated to rely on the built-in health check (removing the prior custom-route example), and tests are adjusted/added to assert /health works by default and returns 404 when disabled.

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