Script API: Player.openContainer to open a container screen

Scripts can read and edit containers, but cannot open one for a player. A container screen opens only on the player's own click, or the inventory key while riding.

Proposal (@minecraft/server):
Player.openContainer(target: Entity | Block): boolean
Player.closeContainer(): void

- The server opens the screen as if the player had interacted with target: same distance, permission and chest_obstruction checks.
- Returns false if the target has no inventory, is too far, or another screen is open (like UserBusy in DDUI).
- New ContainerAccessSource value: Script.
- Works for entities with minecraft:inventory and for blocks, including block_entity containers.

Why it is safe: no packets are exposed and nothing is sent on behalf of the client. The server creates the window itself, with the checks a click already has. Scripts can already seat players, show forms and run commands.

Workarounds today:
1. ActionFormData chest imitations: no real items, no drag.
2. Seating the player on an invisible inventory entity and asking them to press the inventory key.
3. A proxy or server plugin that fakes the client's packet: impossible on Realms.

Related: "Force Interaction With Containers In Scripting API" and "/gui Command".