Add health endpoint

[sfc-gh-jsummer]

This commit adds a endpoint to the MCP server that returns a 200 OK response with a JSON payload {"status": "ok"}. This endpoint enables compatibility with AWS ALB, Kubernetes, and other orchestration systems that require standard HTTP health checks.

Fixes issue 138 and resolves bug in PR 139 by replacing server.app with FastMCP custom-routes.

Health endpoint is only served for http, streamable-http, and sse transports.

[sfc-gh-jsummer]

@laggeduout - Could take another look at your test for the test_health_endpoint.py? You may need to mock the Snowflake connection and/or increase wait time. Currently throws the below error:

requests.exceptions.ConnectionError: HTTPConnectionPool(host='localhost', port=9000): Max retries exceeded with url: /health (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x118fc6e40>: Failed to establish a new connection: [Errno 61] Connection refused'))