Fix exit callbacks not firing on Observer unsubscribe

[C6H15]

When calling Observer:unsubscribe(Group), exit callbacks for entities inside zones aren't called. PR replaces stale cache lookup with a proper check, like how it's already done with group removals.

Test (Client)

--!strict
local Players = game:GetService("Players")

local Player = Players.LocalPlayer

local QuickZone = require(workspace.QuickZone)

local ZonePart = Instance.new("Part")
ZonePart.Anchored = true
ZonePart.CastShadow = false
ZonePart.CanCollide = false
ZonePart.Name = "Test"
ZonePart.Position = vector.zero
ZonePart.Size = vector.one * 20
ZonePart.Material = Enum.Material.ForceField
ZonePart.Color = Color3.fromRGB(0, 0, 255)
ZonePart.Parent = workspace

local Zone = QuickZone.Zone.fromPart(ZonePart)
local Group = QuickZone.Group.localPlayer()
local Observer = QuickZone.Observer.new({
   ["groups"] = {Group},
   ["zones"] = {Zone},
})

Observer:onLocalPlayerEnter(function(Zone)
   local Character = Player.Character
   if Character == nil then return end
   if Character:FindFirstChildOfClass("Highlight") == nil then
   	local Highlight = Instance.new("Highlight")
   	Highlight.FillTransparency = 0.25
   	Highlight.FillColor = Color3.fromRGB(255, 0, 0)
   	Highlight.Parent = Character
   end
end)

Observer:onLocalPlayerExit(function(Zone)
   local Character = Player.Character
   if Character == nil then return end
   local Highlight = Character:FindFirstChildOfClass("Highlight")
   if Highlight ~= nil then Highlight:Destroy() end
end)

task.delay(5, function()
   local Character = Player.Character
   if Character == nil then warn("Character is missing.") return end
   Observer:unsubscribe(Group)
   if Character:FindFirstChildOfClass("Highlight") == nil then
   	print("Observer has unsubscribed, the highlight has been removed.")
   else
   	print("Observer has unsubscribed, but the highlight still exists.")
   end
end)

Test (Server)

--!strict
local QuickZone = require(workspace.QuickZone)

local ZonePart = Instance.new("Part")
ZonePart.Anchored = true
ZonePart.CastShadow = false
ZonePart.CanCollide = false
ZonePart.Name = "Test"
ZonePart.Position = vector.zero
ZonePart.Size = vector.one * 20
ZonePart.Material = Enum.Material.ForceField
ZonePart.Color = Color3.fromRGB(0, 0, 255)
ZonePart.Parent = workspace

local Zone = QuickZone.Zone.fromPart(ZonePart)
local Group = QuickZone.Group.players()
local Observer = QuickZone.Observer.new({
   ["groups"] = {Group},
   ["zones"] = {Zone},
})

Observer:onPlayerEnter(function(Player, Zone)
   local Character = Player.Character
   if Character == nil then return end
   if Character:FindFirstChildOfClass("Highlight") == nil then
   	local Highlight = Instance.new("Highlight")
   	Highlight.FillTransparency = 0.25
   	Highlight.FillColor = Color3.fromRGB(255, 0, 0)
   	Highlight.Parent = Character
   end
end)

Observer:onPlayerExit(function(Player, Zone)
   local Character = Player.Character
   if Character == nil then return end
   local Highlight = Character:FindFirstChildOfClass("Highlight")
   if Highlight ~= nil then Highlight:Destroy() end
end)

task.delay(5, function()
   for _, Player in game:GetService("Players"):GetPlayers() do
   	local Character = Player.Character
   	if Character == nil then warn("Character is missing.") return end
   	Observer:unsubscribe(Group)
   	if Character:FindFirstChildOfClass("Highlight") == nil then
   		print("Observer has unsubscribed, the highlight has been removed.")
   	else
   		print("Observer has unsubscribed, but the highlight still exists.")
   	end
   end
end)