Add a Moonraker backend for Klipper printers

[PyFlat-JR]

What does this PR do?

Adds a Moonraker backend, so Klipper printers running Mainsail or Fluidd work like OctoPrint ones.

• Setup: the address step recognizes Moonraker from /server/info, which OctoPrint does not answer (Moonraker answers OctoPrint's /api/version, so that alone would not tell them apart). No sign-in when this computer is one of Moonraker's trusted_clients, otherwise Moonraker's API key, stored as a secret. Without a name the printer takes the host name of the machine running Klipper.
• Live state: everything after setup goes over Moonraker's JSON-RPC websocket. Klippy's state comes from server.info and the notify_klippy_* notifications, the printer objects from a subscription that only sends changed fields (MoonrakerStatus.Merge).
• Events: Klipper has no job events, so they follow from print_stats.state changing.
• Heaters: extruder, extruder1 and heater_bed, and a chamber as heater_generic chamber (settable) or temperature_sensor chamber (read only).
• Actions: job control, files, temperatures, G-code, motion and system commands (firmware restart, Klipper and Moonraker restart, host reboot and shutdown). G-code waits 1 s for a refusal and then answers sent, since printer.gcode.script only returns once the G-code ran.
• Lights: Moonraker's power devices, as outputs power-<device>. They have no change notification, so they are read every 2 s while there are any.
• Webcam: the first enabled MJPEG webcam Moonraker lists. WebRTC and HLS streams cannot be shown; the address step's advanced settings take an MJPEG address instead.
• Not supported: connecting and disconnecting the printer (Klipper does not do that) and temperature presets. Start and Restart print the last loaded file again, and Print a file always starts the print.
• New backend id moonraker. FakeMoonraker is a Kestrel fake of the HTTP info endpoints and the websocket, and the backend passes the shared BackendContract plus its own MoonrakerTests.
• README (supported table, setup, Privacy) and the manifest description name Moonraker.

Checklist

• I have read AGENTS.md.
• dotnet build and dotnet test pass locally (0 warnings, 188 tests).
• I updated README.md / AGENTS.md if this changes what they describe.
• If this touches a printer backend: I tested it against a real server (see below).

Printer/server tested against (if applicable)

Not yet on a real Klipper printer here. Written against the documented API (https://moonraker.readthedocs.io) and the fake. It ships to a closed tester group with Klipper printers first, through the Creator Portal, before a public release.