fix: don't crash on open_window with an unrecognised window type

[liamnschermerhorn-code]

Opening an unrecognised window type kills the bot outright.

windows.createWindow() returns null when it does not recognise the window type and cannot synthesise a layout — it can only synthesise one when the packet supplies a slot count, and open_window no longer sends slotCount. So any unrecognised type takes that path, and prepareWindow() then dereferences the null:

TypeError: Cannot read properties of null (reading 'id')
    at prepareWindow (lib/plugins/inventory.js:690)
    at Client.<anonymous> (lib/plugins/inventory.js:710)

This is reachable on any server with a mod- or plugin-registered menu, so the bot dies rather than degrading.

Reproduction

Against a NeoForge 26.2 server running Valkyrien Skies Eureka, sneak-right-clicking a ship helm sends:

open_window { windowId: 1, inventoryType: 74, windowTitle: { translate: "gui.vs_eureka.ship_helm" } }

which reliably terminates the bot.

Change

Emit unknownWindow with the raw packet and return, instead of dereferencing the null. Consumers that care about custom menus can listen for it; everyone else simply keeps a live bot. Behaviour for all recognised window types is unchanged.

After the change, the same interaction yields:

[unknownWindow] type=74 title="gui.vs_eureka.ship_helm"
BOT ALIVE: yes

npx standard passes on the changed file.