refactor: move notebooks to notebooks/ with widgets and consistent style

- Rename demo_notebook → notebooks/api_gateway_demo
- Rename test_mcp_agent → notebooks/mcp_gateway_agent_demo
- Add dbutils.widgets.text() with defaults (was missing, caused InputWidgetNotDefined)
- Normalize variable names: APP_HOST, GATEWAY_ID, GENIE_SPACE_ID, TOKEN
- Consistent markdown: intro table, scenario headers with description + code snippet

Author: Rampim