fix(client/context): guard against nil openContextMenu in NUI callbacks

[ASTROWwwW]

Problem

openContext and clickContext index contextMenus[openContextMenu] without checking openContextMenu is set. If a callback fires while no menu is open (closed in between, or a stale message), this is a nil index → Lua error. In openContext, cb(1) runs after the faulty line, so the NUI promise never resolves.

Reproduce

Trigger openContext with back = true (or clickContext) while no context menu is open (e.g. after lib.hideContext()) → attempt to index a nil value.

Fix

Resolve the callback first, then dereference through a nil-checked local in both callbacks